Allow app names to be case-insensitive

Given that application names are lowercase always:

    $  heroku create Foo                                                                                                                     
     !    Name must start with a letter and can only contain lowercase letters, numbers, and dashes.

It's fair that the CLI should force downcasing of app names when using `--app` or `-a`.

I've hit this a few times when taking app names from email/tickets etc that have a leading capital character.