Add text-formatting element and reformat stew.

Only entry-point into wiki extraction methods assumed that the full article was present, which meant you had a bunch of methods (get_title etc.) that would return errors if you for instance wanted to extract text-formatting from just a single section. So now Article class has full-html-level functions but WikiStew class has wiki extractors and can be used to wrap any subsection of HTML. In future, I'll try to make it easier to extract just a single section without parsing the full document first via SoupStrainer functionality of bs4.