add colemak_dh layout

[jamessouth]

adds a layout for colemak_dh as described here
aka colemak_dh_ortho (zxc same as qwerty)
diff with colemak:

< [colemak:layout]
> [colemak_dh:layout]
< g = d
> g = g
< t = g
> t = b
< h = h
> h = m
< v = v
> v = d
< m = m
> m = h
< b = b
> b = v