Caching in isort

[anirudnits]

In reference to https://github.com/PyCQA/isort/pull/1738#issuecomment-850901923 of using caching to improve isort performance. As you said, I would refer to the cache implementation in black and model my implementation based on that and like I did with the multiple config enhancement, I'll open a WIP PR and keep updating and collecting reviews regarding my work. @timothycrosley sounds good?