Add option and/or interactive question to serialize current package versions to a requirements.txt

[Julian Gonggrijp]

Basically a built-in pip freeze >requirements.txt that is run before updating anything. See #96 for rationale.

I personally don't think pip-review needs this functionality, because there is already a command that does the job. Also, it is arguably better to manage dependency versions using pip-tools. Still, if somebody submits a pull request to implement this functionality, I won't stop them.