firmware: serverdown/servertickle should be able to play sub-loops

[todbot]

Currently the "--servertickle" ('D' firmware command) will play the entire programmed color pattern in a loop. It should be possible to play a sub-pattern, like:

blink1-tool --play 1,7,8    # play blue-blue-blue in a loop
blink1-tool -t 3000 --servertickle 1,1,4,5  # after 3 seconds, play green-green-green in a loop

This requires both changes to blink1-tool and the blink(1) firmware.