Add "include-if-exists" functionality

[stefanozoffoli]

Hi and thank you for this library,

i'd like to have a directive that would include a file only if it exists. If it don't exists, it would fail silently.

Something like this:

//=include-if-exists vendor/vendor.js 

It would be really useful, so I could simply delete files that i don't need for a particular project.

Thank you