Add colemak keyboard layout

[Robert Alm Nilsson <robert@robalni.org>]

Hi, I use the colemak keyboard layout but I'm not able to find a
"colemak" option in the installer where you select keyboard layout.
Currently every time I install Debian (which happens pretty often
because I love this operating system) I have to use another keyboard
layout during the installation and then set XKBVARIANT="colemak" in
/etc/default/keyboard after the installation.  It would be awesome
to be able to use colemak from the start.  I hope we can add that as
an option.