Add --next/--prev options to 'send_to_monitor'

This teaches the send_to_monitor command to send a client to either the
next/prev monitor rather than having to always specify a named monitor.
Such options are useful to be able to send clients to monitors relative to
their own position.
Merge pull request #21 from ThomasAdam/ta/send-to-monitor

Add --next/--prev options to 'send_to_monitor'