Implement --freeze-outdated-packages argument

[hueseyincelik]

This adds a new argument --freeze-outdated-packages or -f for saving all outdated packages to a requirements.txt file before upgrading them.

When used in combination with --interactive or -i, only the selected packages are saved.

Resolves #97.

[jgonggrijp]

Thank you for going out of your way to contribute, @hueseyincelik!

As I wrote before in #97, I feel that this way of keeping track of dependencies is probably a bit backwards, but I will still accept the pull request.

Your current implementation holds a subtle breaking change for people who have been forwarding pip's -f option through pip-review in their scripts. See comment below for some additional details. Please fix that; this is my only condition for acceptance.

[hueseyincelik]

Thanks for the quick reply!

Personally, I'm not the biggest fan of this way of tracking package versions either, but since it was trivial to implement, I see no reason not to give users the option.

[jgonggrijp]

In my opinion, "trivial to implement" by itself is not enough justification to do something; it should also be a good idea. In fact, when it is a good idea, it does not even need to be trivial. In any other project, I would probably reject this feature.

For this particular project, I make an exception. pip-review only continues to exist to cater to the people who still want to use it, which no longer includes myself. With hindsight, I agree with @nvie's decision to remove pip-review from pip-tools, so I think I should not impose a "laser focus" and instead let the democratic forces reign free.

[jgonggrijp]

Just published as version 1.3.0: https://pypi.org/project/pip-review/1.3.0/