[TIDOC-1901] Add HTML generators and some refactoring

**Testing**

Default behavior: generate HTML files from the titanium_mobile YAML files 
~~~
node docgen.js
~~~
Prettify it with --css option
~~~
node docgen.js --css templates/htmlejs/styles.css
~~~
Generate module docs -- only generates HTML files from module YAML files
~~~
node docgen.js --css templates/htmlejs/styles.css -f modulehtml ~/projects/appc_modules/ti.facebook/apidoc/
~~~