Do not require file arguments to use -- in CLI invocations

[reese]

For whatever reason, when we rewrote the CLI in clap, the last argument was added to the argument for input files. There's not really a good reason for this -- every other flag is just a bool, so there's no other list args -- and we can still support it without requiring it, which is what this PR does. This also lets users do totally reasonable things like run rubyfmt file.rb and then remember to add a -i, so they can just tack it on the end instead of having to put it back at the beginning.

I've opted to leave the -- in the README examples, since the current version in brew doesn't support not using them, but both with and without -- will now work.