add -o / --output option to write to a file

I can't get the tests to pass on both PY3 and PY2 because of how `click.echo()` has conditional logic based on the Python version. Only PY3 passes currently because it's doing the expected thing. Version `click>=4` will make things work as expected when it gets released.

https://twitter.com/alexconrad/status/542587076563464192