CLI: Only read stdin if there were no files supplied and not in tty mode.

Fixes #563
Ok, to make sure I'm clear, here are the possible invocations that trigger reading from stdin:

* `some_cmd | jscs [options]"
 * not a tty and has no args
* `jscs [options] -`
 * is a tty and has a `-` as the last arg

Am I missing another case?