Add an Extension Point for templating such that other plugins can supply their own template rule instead of ez-templates trying to handle everything itself.

This pre-reqs an amount of maturing of the codebase:

• pipeline support (this changes the interface quite a bit)
• step support (at least a rough idea of how it would look so the public API isn't going to limit us)
• rollback support?

Solution must be:

• migratable (newer ez-templates should continue to support older collaborators with existing in-house code but defer to extensions if they exist)
• backwards-compatible (users can upgrade ez/collaborators independently and expect templating to still work)
  • Investigate how Jenkins uses semantic versioning to see if we can rely on major version changes to block clashing upgrades
• Should use resources (or jelly?) for textual content
• Have unit test which runs in the collaborating project
  • Assert templating works with and without a given exclusion
  • Assert UI text of the exclusion follows any expected pattern
• Documented - JavaDoc mostly then link to it from Confluence