Added multiprocessing support

[bhumijgupta]

Fixes #12

Description The script can now use multithreading. The maximum number of threads are set to 5. Due to this, on local testing the execution time reduced from over 5 minutes to 3:20 .

Other information Threading is only implemented for checking links within the license. The threads still suffer from GIL(Global Interpretor Lock) of python.

Checklist:

• [x] My pull request has a descriptive title (not a vague title like Update index.md).
• [x] My pull request targets the master branch of the repository.
• [x] My commit messages follow best practices.
• [ ] I added tests for the changes I made (if applicable).
• [x] I added or updated documentation (if applicable).
• [x] I tried running the project locally and verified that there are no visible errors.

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[bhumijgupta]

After implementing grequests, the execution time for the script got reduced to 2:50 on Circle-ci.

[bhumijgupta]

A good option would be to squash and merge since the multithreading commit are not needed. This would also help to keep the commit history clean.