[doclint] Implement simple markdown preprocessor

This patch implements simple markdown preprocessor. The goal
is to generate certain parts of markdown, such as:
- puppeteer version
- chromium revision
- table-of-contents
- copy/paste parts of documentation (for shortcut methods)

utils/doclint/preprocessor/test.js

@@ -0,0 +1,48 @@
+/**
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * 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.
+ */
+
+const preprocessor = require('.');
+const SourceFactory = require('../SourceFactory');
+const factory = new SourceFactory();
+const VERSION = require('../../../package.json').version;
+
+describe('preprocessor', function() {
+  describe('gen:version', function() {
+    it('should work', function() {
+      let source = factory.createForTest('doc.md', 'Puppeteer v<!--gen:version--><!--gen:stop-->');
+      let messages = preprocessor([source]);
+      expect(messages.length).toBe(1);
+      expect(messages[0].type).toBe('warning');
+      expect(messages[0].text).toContain('doc.md');
+      expect(source.text()).toBe(`Puppeteer v<!--gen:version-->${VERSION}<!--gen:stop-->`);
+    });
+    it('should tolerate different writing', function() {
+      let source = factory.createForTest('doc.md', `Puppeteer v<!--   gEn:version (  ) -->WHAT
+<!--     GEN:stop   -->`);
+      preprocessor([source]);
+      expect(source.text()).toBe(`Puppeteer v<!--   gEn:version (  ) -->${VERSION}<!--     GEN:stop   -->`);
+    });
+    it('should not tolerate missing gen:stop', function() {
+      let source = factory.createForTest('doc.md', `<!--GEN:version-->`);
+      let messages = preprocessor([source]);
+      expect(source.hasUpdatedText()).toBe(false);
+      expect(messages.length).toBe(1);
+      expect(messages[0].type).toBe('error');
+      expect(messages[0].text).toContain(`Failed to find 'gen:stop'`);
+    });
+  });
+});
+