matrix-spec-proposals

2e5809a6 - Make build.py accept generic files for template var substitutions.

Make build.py accept generic files for template var substitutions. This allows us to incrementally convert sections of the spec to use this templating system. E.g. './build.py ../specification/20_events.rst' where that .rst file has {{room_events}} in it somewhere. Add ability to show a list of valid template vars to use (e.g. room_events) by running './build.py --show-template-vars'.