Deploy an app to Heroku

logical way to handle username/password in an sbt plugin might be 1) allow putting it in some kind of standard-location config file (maybe even shared with `heroku login` command line?) then 2) from command line sbt that config file is used 3) ideally activator edits said config file so configuring this in activator also enables command line usage. I think there are other sbt plugins that do things like this...

Another choice rather than custom config file might be `herokuUser := "foo"` and `herokuPassword := "bar"` sbt settings which could of course be dropped into ~/.sbt/0.13/heroku.sbt. However this may be harder for activator to modify, at least persistently.