Investigate point-release compatibility for extensions

What are the barriers to flagging an extension as compatible with 4.7.13+, for example?

Brain dump:

• Presently the notation in info.xml supports explicit version numbers, not patterns like you see elsewhere in composer.json, etc. (It would be really tedious as an extension maintainer to have to update info.xml for each point release if there are no other code changes.)
• There is presumably some service which tells CiviCRM instances out in the wild which versions of which extensions are compatible with their instance. This would need to be updated.
• We'll need to support the old way of doing things, of course. Can't go breaking the service for people who aren't on the bleeding edge.
• We may want to update the extdir module which fetches release metadata from GitHub, publishes extension releases, etc. to display human-readable version-compatibility strings.

Useful links:

• extdir - module responsible for scraping GitHub and updating the extensions directory
• mock extdir build - buildkit build type which mocks (to some extent) the URL used to refresh the extension cache (e.g., https://civicrm.org/extdir/ver=4.7|cms=Drupal)