api: new method: edit_reviewers

Allow adding and removing reviewers of a pull request with the API call
'edit_reviewers', taking a pull request ID and a list of usernames or
userids to add and remove. For single-user operation, a string can be used
instead of a list.

Note that the 'kallithea-api' tool only accepts strings, so can only perform
single-user operations. Use python 'requests', 'curl' or similar instead if
you need to operate on multiple users at a time.