Clipboard support

[richo]

h/t #65

Old issue was getting derailed.

Thoughts on implementing clipboard generically and having:

tig.clipboard_cmd as a config option to allow interaction with the clipboard?

This would also allow using eg, tmux's buffer.

[jonas]

I am not against this if it is implemented in a way which will be maintainable, however, I'd prefer to fix custom keybindings to be able to handle this use case.