[RP] tab-completions-'r'-us

[Rupert Levene]

Here's a patch that adds a tab-completion mechanism that's fairly
general.

http://rupert.dyndns.org/rp/#tabcomplete

It completes the select prompt with window names, the gselect command
with group names and the colon prompt with aliases and ratpoison
commands. The exec prompt also gets completions by invoking bash,
which is nice nice nice.

It's sleek, shiny and new, so grab it before Shawn makes another
commit that breaks it ;-)

Rupert