Create syntax highlighting for DEP3 patch files

This commit is self-sufficient even though it is the first half of a work I started doing: it adds syntax highlighting for DEP3 headers in .patch files inside a debian/patches directory.

The second half that I'd like to create soon is an omnifunc completion function for the header names and some of the parameter values that they can take.

note: I didn't include a URL link in the file headers since I wasn't sure if they'd make it in the vim-debian repository, but I can add them if needed.