Grabs urls of pictures when scraping post

Fills the (previously unused) Item field "pictures" with urls of any
jpg files in the post. Before being assigned to the Item, it is cast
to a set then back to a list in order to eliminate duplicates.

Duplicates can also come in the form of thumbnails for another image.
It's up to the regular expression in the config json file to exclude
those.

Also, config json changed so that "xpath_rules" are now
"post_parse_rules", and the regex for pictures is under "pictures".