--stdin flag for CLI

[lpil]

Extend the current command line offering with a --stdin flag.

When this flag is given read the input source code from STDIN, format it, and print the outputted style to STDOUT.

This is intended to be used with editor integrations such as https://github.com/sbdchd/neoformat :)

[rgreenjr]

If both the --stdin flag and a path argument are given, we can exit with an error message or ignore the path argument. Do yo have a preference?

[lpil]

I think an error message might be better, but this can be a later improvement if need be.