Easy way to get ready-to-go Heroku::API instance

[danp]

Ref heroku/legacy-cli#508

I currently have the heroku gem in a project's bundle because it has Heroku::Auth.api which makes it really easy to get a ready-to-use Heroku::API instance. I don't have to worry about netrc and other stuff.

It would be nice if heroku.rb itself or another gem outside the main CLI gem provided what is in Heroku::Auth so I could easily get a Heroku::API instance without bringing in the entire CLI.

It's possible this is complicated by plugins that change the auth process.

[geemus]

@dpiddy - how about HEROKU_API_KEY. If that is set then Heroku::API.new should just work I think. Granted it doesn't deal with netrc and the like, but seems fairly close in terms of simplicity. We can possibly do better, but I don't necessarily want to add a netrc dependency here. Maybe it should be a separate little gem that is just a shim for hooking together API with netrc.