Keyboard shortcut to open folder on Windows

Added a keyboard shortcut for the "Open folder" action found in the file menu, based on the behaviour in the Linux version of Atom: Ctrl + Shift + O. Also logical because the keyboard shortcut for the secondary saving option (save as) is set to the original shortcut plus Shift.

PRed after discussion in [this thread at discuss.atom.io](https://discuss.atom.io/t/keyboard-shortcut-to-open-folder-on-windows/13080?u=jpelgrom).