Add import:export command to move revisions between wikis

[samwilson]

e.g. mwcli import:export --wiki from_wiki --towiki to_wiki page_or_category_name (maybe with mwcli export:import being an alias, because the order of wikis might be confusing).

Where page_or_category_name is either a page name or a category name. If a File NS page is given, it should copy the file as well as the page wikitext.

An additional --prefix option could be added that would indicate that the given page_or_category_name is a prefix and that everything under it should be copied.

--dry-run would probably also be useful.