User Retrival: Add possibility to use `continue` parameter

[cutterkom]

Using the continue parameter in user_contributions() allows retrieving more data than limit = 500.

See: https://www.mediawiki.org/wiki/API:Revisions on rvcontinue.

Added a test for it in tests/testthat/test_user_info_retrieval.R

[cutterkom]

Oh, and also a re-built of docs