Create *.mdd.ts extensions which auto-register Markdown directives

[shah]

Now that our Markdown directives capability is sophisticated enough, we should be able to create *.mdd.ts (Markdown directive modules, or "MDD"s) which would auto-register directives and web components they wrap.

This would allow, for example, content/github.mdd.ts to auto-register a GitHub directive which would allow usage like :github[org/repo]{latest} to insert the latest release tag of GitHub org/repo into the current text stream. It would be good to allow these directives to be run at both server and client if desired.

MDDs should wrap web components whenever possible so that HTML and Markdown are both first-class citizens.