Improve dl --select to retrieve releases from github and allow --dev`

[serundeputy]

Right now drush dl --select determines available releases via backdropcms.org update server.

• switch to getting release info via github api
• add a --dev option to get the latest dev HEAD version
• add the dev HEAD version as one of the listed options when using --select option
• add check to default to latest stable, but give dev if there is no release

This will allow users to explore modules that don't have releases yet and easily get the HEAD version if a user knows they need features that are not in a release yet.