Add pyprotoc-plugin as a submodule

[CodingCanuck]

(This issue is sort of a port of 3rdparty/eventuals-grpc#42 [now that the eventuals-grpc repo is read-only] but with a different spin: @while-false please chime in if this approach sounds wrong!)

This repo's version of the pyprotoc-plugin package is stale: we should update it.

Better yet, we should include it as a submodule so that cross-module issues involving pyprotoc-plugin (e.g. https://github.com/reboot-dev/respect-api/issues/17 ) are easier to iterate on within a single locally cloned repo.

[CodingCanuck]

Confirmed offline with @while-false that this is a reasonable change, since pyprotoc-plugin is a dependency of eventuals.

[CodingCanuck]

Added as a submodule in #288