gitcfg: simplify how we get the guitool names

Dynamically find the guitool config options instead of hard-coding their
names.

Related-to: #44

Signed-off-by: David Aguilar <davvid@gmail.com>
cmds: add a way to launch actions in the background

Setting `guitool.<name>.background` will now launch custom commands in
the background, which avoids blocking the UI.

Related-to: #44

Suggested-by: Leho Kraav <leho@kraav.com>
Signed-off-by: David Aguilar <davvid@gmail.com>