Add `exclude` parameter for all collections requests

Each of these requests should support an `exclude` parameter where the value is one or more resource IDs:
- [x] `GET /wp/v2/posts`
- [x] `GET /wp/v2/comments`
- [x] `GET /wp/v2/users`
- [x] `GET /wp/v2/<taxonomy>`

Related #1668, #924 
