Add an option to replace imports

[hamdanal]

I am trying my luck with #1534. I am not familiar with isort internals though so I don't know how I am doing. I appreciate if you could take a look at this PR and advise me on how to proceed.

Some context:

• Currently, the new feature passes simple unit tests. I believe more tests must be added but I would like to get some feedback before proceeding.
• No handling of action comments as of yet