Deploy an app to Heroku

this one seems straightforward `herokuConfigGet` / `herokuConfigSet` *except* I think the current input task mechanism would result in an annoying need to escape what's passed to sbt. So it would be nicer to be able to at least give sbt an "argv" (already-unquoted) but maybe even cleaner would be if an input task parser could be a `String => ConfigMap => action` function where it's possible to call only the `ConfigMap => action` part with no string. @jsuereth this is another architectural problem for us.