Support fully customized command-line key maps

[mtalexan]

A continuation of #8, allow for more than just hard coded key maps.
I'm not experienced with X11, so I didn't realize there was XStringToKeysym() as an option.

Instead of hard coding values from keysym.h to define a map, require a minimum sized map be provided on the command line if they specifiy --keymap=custom (or something similar), and allow a user to specify the names of keys that can be passed to XSTringToKeysym() as a comma-separated list (or something like that?).

This would require the map function to allow for a dynamically determined map size rather than a pre-defined map size, which is currently a non-trivial task.