feat(core): Implement footer-contains rules

@commitlint/core/src/library/ensure-contains.js

@@ -0,0 +1,9 @@
+export default (value, regex) => {
+	if (value === undefined) {
+		return false;
+	}
+	if (!regex instanceof RegExp) {
+		return false;
+	}
+	return regex.test(value);
+};

@commitlint/core/src/library/ensure-contains.test.js

@@ -0,0 +1,17 @@
+import test from 'ava';
+import ensure from './ensure-contains';
+
+test('false for no params', t => {
+	const actual = ensure();
+	t.is(actual, false);
+});
+
+test('true for /^foo/gi against foo', t => {
+	const actual = ensure('foo', /^foo/gi);
+	t.is(actual, true);
+});
+
+test('false for /^foo/gi against notfoo', t => {
+	const actual = ensure('notfoo', /^foo/gi);
+	t.is(actual, false);
+});

@commitlint/core/src/rules/footer-contains.js

@@ -0,0 +1,20 @@
+import ensureContains from '../library/ensure-contains';
+import message from '../library/message';
+
+export default (parsed, when, value) => {
+	if (!parsed.scope) {
+		return [true, ''];
+	}
+
+	const negated = when === 'never';
+	const result = value.length === 0 || ensureContains(parsed.scope, value);
+
+	return [
+		negated ? !result : result,
+		message([
+			`footer content must`,
+			negated ? `not` : null,
+			`pass the regular expression: ${value.toString()}`
+		])
+	];
+};

@commitlint/core/src/rules/footer-contains.test.js

@@ -0,0 +1,33 @@
+import test from 'ava';
+import parse from '../library/parse';
+import footerContains from './footer-contains';
+
+const messages = {
+	empty: 'foo(bar): baz',
+	matched: 'foo(bar): baz\n\nbody\n\nqux',
+	unmatched: 'foo(bar): baz\n\nbody\n\nquux'
+};
+
+const parsed = {
+	empty: parse(messages.empty),
+	matched: parse(messages.matched),
+	unmatched: parse(messages.unmatched)
+};
+
+test('footer-contains with no footer should not succeed', async t => {
+	const [actual] = footerContains(await parsed.empty, 'always', []);
+	const expected = false;
+	t.deepEqual(actual, expected);
+});
+
+test('footer-contains with matching footer should succeed', async t => {
+	const [actual] = footerContains(await parsed.matched, 'never', []);
+	const expected = true;
+	t.deepEqual(actual, expected);
+});
+
+test('footer-contains with non-matching footer should not succeed', async t => {
+	const [actual] = footerContains(await parsed.unmatched, 'never', []);
+	const expected = false;
+	t.deepEqual(actual, expected);
+});

@commitlint/core/src/rules/index.js

@@ -5,6 +5,7 @@ export default {
 	'body-max-length': require('./body-max-length'),
 	'body-min-length': require('./body-min-length'),
 	'body-tense': require('./body-tense'),
+	'footer-contains': require('./footer-contains'),
 	'footer-empty': require('./footer-empty'),
 	'footer-leading-blank': require('./footer-leading-blank'),
 	'footer-max-length': require('./footer-max-length'),