Add CI to Pull Requests

[mburns]

Github lets repositories protect branches and lock Pull Requests from being mergable until status API says everything is good. We can use this to only let a PR be merged once it is well-formatted and has enough votes to pass.

• validate Markdown file formatting
• non-duplicate Rule and RFJ numbers
• metadata validation of rules/RFJ
  • filename numbers match
  • status/authorship is correct