Eliminate dump-screen command

The test suite this was written for never materialized and I'd take a
different approach now anyway.
Fix color config validator
Hook up color command

The syntax has changed. Pair can be numeric, or deffg/defbg (in which
case only the first color argument is taken). The colors can be numeric
0 - 255, or common curses aliases (red, cyan, blue, green, yellow,
magenta/pink, black, white).

:color [pair] [fg] [bg]