Open last capture location (directory)#158

Add an item to tray menu that would open the directory where the last captured file (screenshot / screencast) was saved. Especially if Autosave is off, this may be a non-standard location.

My use case is capturing & saving several screenshots, and then opening Finder to manipulate the files further (compare, upload, etc.)

I am using Monosnap on macOS, but I assume this may be applicable for other platforms as well.