Adds ability to specify config file.

[vtrlx]

This patch adds the -c option to the command-line, allowing the user to specify another location for the config file. Ex:

zx -c ~/.config/i3/zx.conf

These changes still allow one to override config options on the command-line e.g:

zx -c ~/.config/i3/zx.conf -d 0

will disable daemon mode regardless of what ~/.config/i3/zx.conf specifies.