Offer dev branch for downloading in case if there is no official release

[alanmels]

Running the drush dl pwa command gives:

docker@cli:/var/www/docroot/modules$ drush dl pwa

	Error: There is no release located at https://github.com/backdrop-contrib/pwa/releases/download//pwa.zip.
	Try visiting the pwa page directly: https://github.com/backdrop-contrib/pwa

docker@cli:/var/www/docroot/modules$ 

Indeed there is no release for the module at https://github.com/backdrop-contrib/pwa/releases. However, there is a development branch 1.x-1.x with a zip-file at https://github.com/backdrop-contrib/pwa/archive/1.x-1.x.zip. So instead of explicitly setting the download address to https://github.com/backdrop-contrib/$project/releases/download/$latest[0]/$project.zip, wouldn't be nice if drush offered the development branch for downloading just to cover modules with no official releases?

[serundeputy]

@alanmels this is a great idea and will be handled in #107

closing as duplicate.