Fetch packages from requirements.txt

[Aposhian]

Is your feature request related to a problem? Please describe.
I would like to use the output of pip freeze directly with the unearth cli.

Describe the solution you'd like
Add a -r, --requirements flag that takes a requirements file, and then operates on all the package descriptors in the file.

Describe alternatives you've considered
Writing a script to invoke unearth many times.

Additional context
pip download "supports" this (despite all its flaws).

[frostming]

It sounds good to me. Just that unearth won't do any dependency resolution, unlike pip. So child dependencies won't be fetched(like pip download --no-deps).

However, I will not get to this shortly. Feel free to take this and open a PR.