Added the skeleton of a Groovy DSL.

Anyone interested in #13 is invited to submit PR's against branch feature/groovy.  Spotless' maintainers are not fans of Groovy.  We'll help and support anyone who wants to contribute, but we don't actually know anything about it.

src/main/java/com/diffplug/gradle/spotless/SpotlessExtension.java

@@ -23,6 +23,7 @@
 import org.gradle.api.Project;
 
 import com.diffplug.gradle.spotless.freshmark.FreshMarkExtension;
+import com.diffplug.gradle.spotless.groovy.GroovyExtension;
 import com.diffplug.gradle.spotless.java.JavaExtension;
 import groovy.lang.Closure;
 
@@ -59,6 +60,19 @@ public void java(Consumer<JavaExtension> closure) {
 		closure.accept(java);
 	}
 
+	/** Configures the special groovy-specific extension. */
+	public void groovy(Closure<GroovyExtension> closure) {
+		GroovyExtension groovy = new GroovyExtension(this);
+		closure.setDelegate(groovy);
+		closure.call();
+	}
+
+	/** Configures the special groovy-specific extension. */
+	public void groovy(Consumer<GroovyExtension> closure) {
+		GroovyExtension groovy = new GroovyExtension(this);
+		closure.accept(groovy);
+	}
+
 	/** Configures the special freshmark-specific extension. */
 	public void freshmark(Closure<FreshMarkExtension> closure) {
 		FreshMarkExtension freshmark = new FreshMarkExtension(this);

src/main/java/com/diffplug/gradle/spotless/groovy/GroovyExtension.java

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2016 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.gradle.spotless.groovy;
+
+import java.util.List;
+
+import org.gradle.api.GradleException;
+import org.gradle.api.internal.file.UnionFileCollection;
+import org.gradle.api.plugins.JavaPluginConvention;
+import org.gradle.api.tasks.SourceSet;
+
+import com.diffplug.gradle.spotless.FormatExtension;
+import com.diffplug.gradle.spotless.FormatTask;
+import com.diffplug.gradle.spotless.LicenseHeaderStep;
+import com.diffplug.gradle.spotless.SpotlessExtension;
+import com.diffplug.gradle.spotless.java.ImportSorterStep;
+
+// TODO: None of this is actually groovy specific.  Spotless'
+// maintainers are not fans of Groovy, but we're happy to take
+// PRs to flesh out this functionality.
+public class GroovyExtension extends FormatExtension {
+	public static final String NAME = "groovy";
+
+	public GroovyExtension(SpotlessExtension rootExtension) {
+		super(NAME, rootExtension);
+	}
+
+	public static final String LICENSE_HEADER_DELIMITER = "package ";
+
+	public void licenseHeader(String licenseHeader) {
+		licenseHeader(licenseHeader, LICENSE_HEADER_DELIMITER);
+	}
+
+	public void licenseHeaderFile(Object licenseHeaderFile) {
+		licenseHeaderFile(licenseHeaderFile, LICENSE_HEADER_DELIMITER);
+	}
+
+	public void importOrder(List<String> importOrder) {
+		customLazy(ImportSorterStep.NAME, () -> new ImportSorterStep(importOrder)::format);
+	}
+
+	public void importOrderFile(Object importOrderFile) {
+		customLazy(ImportSorterStep.NAME, () -> new ImportSorterStep(getProject().file(importOrderFile))::format);
+	}
+
+	/** If the user hasn't specified the files yet, we'll assume he/she means all of the java files. */
+	@Override
+	protected void setupTask(FormatTask task) throws Exception {
+		if (target == null) {
+			JavaPluginConvention javaPlugin = getProject().getConvention().getPlugin(JavaPluginConvention.class);
+			if (javaPlugin == null) {
+				throw new GradleException("You must apply the java plugin before the spotless plugin if you are using the java extension.");
+			}
+			UnionFileCollection union = new UnionFileCollection();
+			for (SourceSet sourceSet : javaPlugin.getSourceSets()) {
+				union.add(sourceSet.getJava());
+			}
+			target = union;
+		}
+		// LicenseHeaderStep completely blows apart package-info.java - this common-sense check ensures that
+		// it skips package-info.java.  See https://github.com/diffplug/spotless/issues/1
+		steps.replaceAll(step -> {
+			if (LicenseHeaderStep.NAME.equals(step.getName())) {
+				return step.filterByFile(file -> !file.getName().equals("package-info.java"));
+			} else {
+				return step;
+			}
+		});
+		super.setupTask(task);
+	}
+}