Add support for the new tags format with ghost 0.4
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.

mattvh/Jekyll-to-Ghost