Add possibility to add keyboard shortcut

[skoglump]

In the settings it should be an option for setting a keyboard shortcut. When that keyboard combination is hit it should be the same result as pressing the coffee cup (enable or disable)

[coadmunkee]

I have a working prototype version with hotkey support available in the feature-hotkey branch. If you want to test it, you can retrieve that branch and install it following the manual installation instructions on the README page. By default you should be able to use Ctrl+Super+E to toggle Espresso on and off. There is also an option currently tagged on the bottom of the Preferences window that lets you pick a different key combination.