Address over-eager fallback to stdin when decoding

[roberth-k]

On MacOS 10.12.6 using iTerm2 and Bash, the condition at https://github.com/jpadilla/pyjwt/blob/master/jwt/__main__.py#L57 appears to always evaluate to True regardless of whether the user had passed a JWT on the command line or not. Hence a command such as pyjwt decode --no-verify json.web.token (where json.web.token is a valid JWT) is not usable.

This change modifies the command-line interface such that the application will only attempt to read from stdin if the input had not been passed.

[coveralls]

Coverage decreased (-1.9%) to 98.077% when pulling 5a6b783 on milliburn:fix-decode-read-stdin into 608ed4a on jpadilla:master.

[jpadilla]

@milliburn thanks for getting this done. Confirmed this via #298 so just opened #304 to get this merged. Your original commit is still attributed to you.