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

Fixes #563
Restoring `process.stdin.on` is problematic. I'm not 100% sure why (still trying to find the cause through sinon's source), but `process.stdin.on.restore` only exists until the first call to `process.stdin.on` at https://github.com/mrjoelkemp/node-jscs/blob/fix_stdin/lib/cli.js#L117. After this call, `process.stdin.on.restore` is gone. Hence, adding a restore call in the promise callback throws an error.