Provides syntax highlighting for MacPorts Portfiles.

This code was copied from the 'macports-contrib' repository
(https://github.com/macports/macports-contrib) at commit
513c455d5a902fff2b38edf86abc15c36688bf85
(https://github.com/macports/macports-contrib/tree/513c455d5a902fff2b38edf86abc15c36688bf85).

The Vim plugin lives in a subdirectory of the 'macports-contrib'
repository, so I'd have to include a whole mess of files that don't
relate to Vim if I wanted to add the plugin as a submodule. I'd just as
soon not have to copy the files into my repository tree like this, but
it's better than adding a non-Vim submodule.

Assuming it's okay to add these files to my repository, as the port file
for this plugin describes it as being BSD licensed:

https://github.com/macports/macports-ports/blob/12d7b1597bef875058758c9c587b7593f39a5af1/editors/mpvim/Portfile

Unfortunately, there's no BSD license or copyright notice included in
the plugin code. But I'm going to assume I'm allowed to redistribute it.