Introduce /exec support for running simple client-side scripts

Introduce /exec support for running simple client-side scripts

/exec myscript [param1 param2 ...] runs an executable named "myscript" in one of the acceptable
script dirs ($configdir/scripts, $datadir/scripts) and pipes its stdout into IRC. This is not much,
but enough to run stuff like infobash/inxi or the infamous now-playing stuff :P

It's recommended to use aliases for invoking scripts, e.g. /audio could expand to /exec nowplaying,
where nowplaying is your fancy script that queries your Amarok for the current song.

Now keep 'em scripts coming, good ones will be included in our repo :)