Remove trailing , from tags
Add support for the new tags format with ghost 0.4

ghost 0.4 now stores tags "properly" in a posts_tags table which maps
the post_id with the tag_id

Note: ghost 0.4 has a bug where it discards everything but one tag for
each post. Tested this by manually adding tags - exporting via ghost and
importing via ghost.
Convert all tags to lowercase

Because I have a tag called "KDE" and "kde", and both since both their
slugs are the same it cannot be inserted.