No strong feeling on landing this, but just wanted to see how our plugin
would look if we integrated https://github.com/bufbuild/protoplugin,
which eliminates some of the boilerplate of handling the plugin.

Vaguely relates to #18, although I think the language the plugin is
written in now is less important than ever.

Also takes a pass at cleaning up the go code and tests.

Signed-off-by: Stefan VanBuren <svanburen@buf.build>

Signed-off-by: Stefan VanBuren <svanburen@buf.build>