Should check existence of file when interpolate ${project_path:}

As the comment in [CompleteSharp.sublime-settings](https://github.com/quarnster/CompleteSharp/blob/master/CompleteSharp.sublime-settings) says, when `${project_path:file_name}` doesn't find any thing, it should return `file_name`. 

I think this behavior makes perfect sense, so could you re-add this into master?