Print help args if running rubyfmt with no args in tty

[reese]

Resolves #373

rubyfmt with no args generally reads from stdin, but some users will just run it on its own. There are several things we could do here, like just exit early, return what would be formatted by passing "", or something else, but I think printing the equivalent of rubyfmt --help is the most user-friendly (although I'm open to an alternative, since many unix tools will just return "" or something like that).

I tried adding a test for this, but I'm terrible with bash and faking tty in our bash tests was getting to be more effort than it's worth, so I opted to just stick with manual testing with cargo run and making sure that echo "class Foo; end" | cargo run still works as expected.