Remove call to mf2py.Parser.__doc__. Pass in a BS4 object instead
Posse-post-discovery: added util.ensure_content_type to check
content-type headers before fetching arbitrary URLs.

Addresses #158
Replace ensure_content_type with calls to get_webmention_target.

The two methods were doing essentially the same thing.
Merge pull request #159 from kylewm/ppd-check-content-type

posse-post-discovery: check for content-type text/html before fetching h-feeds and permalinks