RFC - Add Indie and Pro license 'install' commands

[Nitemaeric]

Streamline DR installation by allowing users to download Indie and Pro licenses from their command line.

New commands

• drenv login

  This requires authentication tokens to be implemented on https://dragonruby.org. We should never store a user's password in plaintext, even on their own device.

• drenv install <version>

  This command will prompt a user to enter their credentials (as we currently don't have any authentication tokens to cache) and download a version of DragonRuby and register it to ~/.drenv/versions.

  e.g.

  $ drenv install 6.3-indie

  $ drenv install 6.3-pro

Considerations

• Right now, the DR download API only supports downloading the latest version. We'd probably need to talk to Amir why that is and if we can request specific versions.
• I've looked into Itch.io's API docs and it doesn't look like there is a way to download files via API anymore, unless you're the project owner. This limitation means that I'm unable to add support for downloading the Standard tier versions. Another thing that Amir could possibly support by providing Standard downloads from https://dragonruby.org itself.