Add support for multiple categories in exported requirements

Now that Pipfile supports multiple categories, it would be nice if I could create different requirements files for these categories. Let’s say my Pipfile has categoires: tests, documentation, foo, bar. I would like to create the following files: requirements-tests.txt, requirements-documentation.txt, requirements-foo.txt and requirements-bar.txt.

I would like to export them with:

pipenv requirements --category tests > requirements-tests.txt

And so on …

However, right now only these options are supported:

 $ pipenv requirements -h
Usage: pipenv requirements [OPTIONS]

Options:
  --dev              Also add development requirements.
  --dev-only         Only add development requirements.
  --hash             Add package hashes.
  --exclude-markers  Exclude markers.
  -h, --help         Show this message and exit.

[BAllbeeDHI]

Thank you for taking that on, @dojutsu-user — Looking forward to having that capability as soon as it’s released. 😄

[dojutsu-user]

HI @oz123, Raised a PR with probable fix. Can you please check it once if the approach looks fine to you.