Add cli arg to load config from custom location

[ProbablyRohan]

Add a command line argument to (e.g. -c) to allow users to load a config file from anywhere instead of just ~/.config/bwm/config.ini.

example:

$ bwm -c /path/to/config

[firecat53]

Yep, I need to port this over from Keepmenu. Thanks for the prompt.