Add 'show in file manager' to active document context

This adds the "Show in File Manager (/Explorer/Finder)" to the context menu of the active document - a feature from my ex-editor that I used a lot. 
![showinfinder](https://cloud.githubusercontent.com/assets/129330/3070543/8d8393a2-e2aa-11e3-9694-ef6db410603e.png)
It works by running the `revealActiveFile`, followed by `showSelectedEntryInFileManager`. If it's not the kind of thing you're looking for, then feel free to close this guy.