Add recipe: Vala-mode

Recipe for the vala-mode for editing vala source code.

Note that the project is not mine, and is not officially maintained in any version control system, just a downloadable file here: https://wiki.gnome.org/Projects/Vala/Emacs and since there's no direct url fetcher I pointed it to my github, I guess this counts as an 'extreme' circumstance?

Build and installed worked.