Add support for marker functions, to be able to extract strings not directly passed to TranslateService. Closes #10

tests/parsers/function.parser.spec.ts

@@ -0,0 +1,27 @@
+import { expect } from 'chai';
+
+import { FunctionParser } from '../../src/parsers/function.parser';
+
+describe('FunctionParser', () => {
+
+	const componentFilename: string = 'test.component.ts';
+
+	let parser: FunctionParser;
+
+	beforeEach(() => {
+		parser = new FunctionParser();
+	});
+
+
+	it('should extract strings using marker function', () => {
+		const contents = `
+			import { _ } from '@biesbjerg/ngx-translate-extract';
+			_('Hello world');
+			_(['I', 'am', 'extracted']);
+			otherFunction('But I am not');
+		`;
+		const keys = parser.extract(contents, componentFilename).keys();
+		expect(keys).to.deep.equal(['Hello world', 'I', 'am', 'extracted']);
+	});
+
+});