Allow to update multiple requirements files

[gdalmau]

Description

I was using https://github.com/simion/pip-upgrader for a while and having the following structure:

requirements/
    base.txt
    dev.txt
    prod.txt

I could update all of them using pip-upgrade requirements/*.txt.

It would be nice that pur allows the same to update multiple requirements files.

Workaround

for filename in requirements/*.txt; do pur -r $filename; done