Allowing for custom blacklist

Hi,

Thanks a lot for this amazing extension to spaCy. I have used it in a project and it was very helpful.

I was wondering if it is possible to add a feature where one can pass a custom list for the blacklist parameter.

For instance:
If blacklist is enabled
```
nlp("I really liked that ice cream. My dog did not.")._.coref_resolved
```
returns "I really liked that ice cream. My dog did not." unchanged.

However, 'we', 'out', 'us' are not in the blacklist, so,
```
nlp("We really liked that ice cream. Our dog did not.")._.coref_resolved
```
returns "We really liked that ice cream. We dog did not."

It would be very helpful if it was possible to customise the blacklist.

Start by locating the blacklist parameter and the coref_resolved extension described in the examples. Check how blacklist values are configured and applied, then define the custom-list interface and verify that supplied terms remain unchanged while existing blacklist behavior is preserved.

Written by the indexing model from the issue text.