Added support for a .xinitrc-like file (.westonrc)

Hello!

I was looking for a way to auto-start a few apps and services after Weston, but I couldn't find one, so I thought about making this patch. It has been tested and is simple enough that it shouldn't introduce any problem, but please let me know if there's anything that should be changed.

The file location is hardcoded to $HOME/.westonrc, and /bin/sh is used to execute it (basically it emulates the system function but gives us a bit more of control). I have also added a note about its existence to weston(1), and added the build/ directory to the .gitignore, since the README uses it to demonstrate the build process.

Thanks for reviewing this.

Signed-off-by: Alvarito050506 donfrutosgomez@gmail.com