Add support for hashtags (TagPage-s), i.e. explore/tags/<tag> URLs.
This also introduce a get_metadata() method in order to append
possible further metadata per-(sub)extractor.
Refactor and generalize _extract_profilepage() to _extract_page()
in order to be reused by _extract_profilepage() and _extract_tagpage()
simply by passing the type of page (`ProfilePage' or `TagPage') and picking up
the respective fields in shared data.
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