IPC (external scripting support)

This PR allows howm to be controlled over a UNIX socket. I have written a small utility to interact with howm called [cottage](https://github.com/HarveyHunt/cottage).

I think there is an issue with the use of ```args``` around line 3005- when using valgrind, the pointer ```args``` isn't correctly advanced.

Feedback on code and docs would be great. :-)