Make it possible to start the server in socket mode

This launcher makes possible to start LS in socket mode for better debug
experience.
1. Run the XMLServerSocketLauncher in debug mode (e.g. in eclipse).
2. Connect via socket port

Theia language contribution example:
```
            let socketPort = '5008'
            console.log(`Connecting via port ${socketPort}`)
            const socket = new net.Socket()
            const serverConnection = createSocketConnection(socket,
socket, () => {
                socket.destroy()
            });
            this.forward(clientConnection, serverConnection)
            socket.connect(socketPort)
```


Signed-off-by: Dennis Huebner <[email protected]>

README.md

@@ -47,6 +47,29 @@ Get started
 * Run `./mvnw clean verify` (OSX, Linux) or `mvnw.cmd clean verify` (Windows)
 * After successful compilation you can find the resulting `org.eclipse.lsp4xml-all.jar` in the folder `org.eclipse.lsp4xml/target`
 
+Developer
+--------------
+
+To debug the XML LS you can use XMLServerSocketLauncher:
+
+1. Run the XMLServerSocketLauncher in debug mode (e.g. in eclipse)
+2. Connect your client via socket port. Default port is 5008, but you can change it with start argument `--port` in step 1
+
+Client connection example using Theia and TypeScript:
+
+```
+            let socketPort = '5008'
+            console.log(`Connecting via port ${socketPort}`)
+            const socket = new net.Socket()
+            const serverConnection = createSocketConnection(socket,
+                socket, () => {
+                    socket.destroy()
+                });
+            this.forward(clientConnection, serverConnection)
+            socket.connect(socketPort)
+```
+
+
 Clients
 -------
 

org.eclipse.lsp4xml/src/main/java/org/eclipse/lsp4xml/XMLServerSocketLauncher.java

@@ -0,0 +1,73 @@
+/**
+ * Copyright (c) 2018 TypeFox GmbH (http://www.typefox.io) and others.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ * 
+ * @author Dennis Huebner <[email protected]> - Initial contribution and API
+ */
+package org.eclipse.lsp4xml;
+
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.net.InetSocketAddress;
+import java.nio.channels.AsynchronousServerSocketChannel;
+import java.nio.channels.AsynchronousSocketChannel;
+import java.nio.channels.Channels;
+import java.util.Objects;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import org.eclipse.lsp4j.jsonrpc.Launcher;
+import org.eclipse.lsp4j.jsonrpc.MessageConsumer;
+import org.eclipse.lsp4j.services.LanguageClient;
+
+public class XMLServerSocketLauncher {
+
+	private static final int DEFAULT_PORT = 5_008;
+
+	/**
+	 * Calls {@link #launch(String[])}
+	 */
+	public static void main(String[] args) throws Exception {
+		new XMLServerSocketLauncher().launch(args);
+	}
+
+	/**
+	 * Launches {@link XMLLanguageServer} using asynchronous server-socket channel and makes it accessible through the JSON
+	 * RPC protocol defined by the LSP.
+	 * 
+	 * @param args standard launch arguments. may contain <code>--port</code> argument to change the default port 5008
+	 */
+	public void launch(String[] args) throws Exception {
+		AsynchronousServerSocketChannel _open = AsynchronousServerSocketChannel.open();
+		int _port = getPort(args);
+		InetSocketAddress _inetSocketAddress = new InetSocketAddress("0.0.0.0", _port);
+		final AsynchronousServerSocketChannel serverSocket = _open.bind(_inetSocketAddress);
+		while (true) {
+			final AsynchronousSocketChannel socketChannel = serverSocket.accept().get();
+			final InputStream in = Channels.newInputStream(socketChannel);
+			final OutputStream out = Channels.newOutputStream(socketChannel);
+			final ExecutorService executorService = Executors.newCachedThreadPool();
+			XMLLanguageServer languageServer = new XMLLanguageServer();
+			final Launcher<LanguageClient> launcher = Launcher.createIoLauncher(languageServer, LanguageClient.class,
+					in, out, executorService, (MessageConsumer it) -> {
+						return it;
+					});
+			languageServer.setClient(launcher.getRemoteProxy());
+			launcher.startListening();
+		}
+	}
+
+	protected int getPort(final String... args) {
+		for (int i = 0; (i < (args.length - 1)); i++) {
+			String _get = args[i];
+			boolean _equals = Objects.equals(_get, "--port");
+			if (_equals) {
+				return Integer.parseInt(args[(i + 1)]);
+			}
+		}
+		return DEFAULT_PORT;
+	}
+}