feat: allow skipping requirements file includes

[Raekkeri]

Hi,

I'm building a tool to inspect requirement files of multiple Python projects (compare version numbers etc.). Your library works very nicely, and I'd like to propose this small update using which the requirements file "includes" ("-r" or "--requirement") can be ignored. Let me know if you're interested in applying this or if you'd like to have anything changed/improved in the PR.