add windowfocus-follows-mouse feature

[Quoteme]

just as the title says. There should be an equivalent to focus_follows_mouse (yes|no) as in i3. So if you move the mouse from one window into another, the focus should switch to that window.
Default should be set to yes, because thats how it is in i3

[aesophor]

Hi there, this feature is now available in the staging branch! #34

Currently it seems to be bug-free (what a bold statement lol), but I'll test it for a few days. If everything goes well, I'll merge it into master.

[aesophor]

@Quoteme @the-coffee-coder

The focus_follows_mouse feature has been merged into master! 😄