Add a configurable keymap

Multiple keymaps may be added in either Gertty itself or in config
files, individual keys may be overriden in the standard map, and
the map can be selected via config file or command line option
just as palettes are.

Change the help text to be dynamically generated based on the
current keymap.

Change-Id: I5f8e63897fab3aa14493465256b5d4516cf47dcd