Add ability for an Ignore list

[vraravam]

This is a great project - thank you!

I would like to request a new feature: Ability to add an ignore list

In our project, we have a lot of bots that show up when I choose to Compare contributors from the repository with the credited ones from the cli. I would like the ability to specify this list in an .all-contributorsrc-ignore (or any other filename) such that this list need not be perused manually everytime to "remove" those entries.

[Berkmann18]

That seems sensible, how would you go about implementing that?

cc @all-contributors/core