Refactor the tpl engines sub-generators

There's lotta copy/paste in there.

For the generators I was thinking to use the same principle as I did for the task runners sub-generators. 
Extending a base: https://github.com/SassDoc/generator-sassdoc-theme/blob/master/lib/taskrunner-base.js

The theme `index.js` templates, maybe we could have some sub-templates. Tricky because the differences are few, but spread over the file.
