Allow CKAN files to be generated from github releases

[pjf]

The CKAN meta-file requires that a version and a download link be provided; this allows simple installers and mirror bots (which must check the license field) to be written easily. However it's a pain having to update the download URL and version every time one makes a release.

A potential solution is to provide a partial META.json file in a distribution, with some sort of magic releases flag enabled. This signals to external tools that the file contents should be combined with the release info from github (accessible via the API) to produce the final file. This means authors can continue to make releases as normal, and the CKAN index can do the hard work of filling in version numbers, download links, and changelog entries.

It should be noted that the resulting CKAN file can't be included in the released zip, since that zip has already been created and released. That's okay, because bundling CKAN files isn't required for operation, but it's something we encourage folks to do to allow for tools to more easily detect which mods are installed.

[pjf]

We probably need a way of specifying a regexp that will be used to match release tags. This means we can detect the difference between two mods using the same repo (eg: NEAR and FAR), and betwen devel and stable versions being released (as happens with RemoteTech).