Generate appup files for hot upgrades and downgrades

Examples

# Generate an appup for :test from the last known version to the current version
mix release.gen.appup --app=test

# Generate an appup for :test from a specific version to the current version
mix release.gen.appup --app=test --upfrom=0.1.0

The generated appup will be written to rel/appups/<app>/<from>_to_<to>.appup. You may name appups anything you wish in this directory, as long as they have a .appup extension. When you build a release, the appup generator will look for missing appups in this directory structure, and scan all .appup files for matching versions. If you have multiple appup files which match the current release, then the first one encountered will take precedence, which more than likely will depend on the sort order of the names.

This task will take all of the same flags as mix release, but only uses them to determine the release configuration to use when determining application locations and versions.