WordPress: fixed import and enabled post types

This replaces pull request https://github.com/chyrp/chyrp/pull/194

Fixes:
* enable imports from wordpress.com
* tag import
* category import
* check wp:post_type to prevent processing of attachment posts
* correct media url replacement

Enable post types:
* add module that imports typed Wordpress posts (link, quote, video, image) to the right Chyrp feathers (link, quote, video, photo) if they are enabled. Other/unknown post types are still imported as text feather.