Add ability for an Ignore list

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.