Supporting custom file extensions

Hola!

* enabled js, css, html and jade by default
* you can specify which file extensions to rev with `options.fileExt`
* added tests to verify that you can exclude/include file extensions at will

Long story short, we use express and use the view cache, so that
jade templates are compiled upon the first request.
At the same time we also need to rev some file on those jade files,
but gulp-rev-all doesnt rev references in them as it only supports
html|css|js, and it's a bit too strict :)