[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)

docs/api.md

@@ -1,4 +1,4 @@
-# Puppeteer API
+# Puppeteer API v<!-- GEN:version -->0.1.0<!-- GEN:stop-->
 
 ##### Table of Contents