Don't attempt to start xmobar unless we know its config file exists and don't
use hardcoded home directories.

Note: Might also be good to check if xmobar is in the $PATH too.
Safely use Option for find_screen, no longer asuming Option is Some. It is very
risky to use Option::unwrap(). Patching: #33 and #40.
Merge pull request #41 from james-darkfox/patch-3

Patch: #33 and #40; No longer assuming Option is Some for unwrap() for find_screen.
Merge pull request #34 from james-darkfox/patch-1

Check if xmobar_config exists