Create compliance framework for mu extensions

[PaulDuvall]

1. A wiki checklist that includes ease of use of the README and other requirements for the extension
2. A mu subcommand that creates the scaffolding for a mu extension: GitHub repo (and name), README, mu-extensions.yml, the mu asset to extend, LICENSE, etc.
3. A service (e.g. API) that tests a mu extension with an existing mu example. Could be configured to run with every GitHub release or on a schedule. This could be configured to update a badge displayed in the README of a repository (e.g. using https://shields.io/). Might also indicate last time it was tested and the current pass/fail status via the badge or other mechanism.