[FEATURE] Generate appcast.xml file from <tool>-versions.json

- Access to /releases/<tool>/appcast.xml dynamically
  generates a appcast.xml file which includes (only!) the latest
  version of a tool.
  (Caching to be added later.)
- Adds XML rendering support to LLView if the view name is suffixed
  with :xml
- Helper function newest_version_from_versions can be used to determine
  the newest version from a versions array.
Merge branch 'new' of https://github.com/GPGTools/GPGTools_Homepage into new