Fix prefixed image options, implement link=

See e.g. https://www.mediawiki.org/wiki/Parsoid/MediaWiki_DOM_spec#Simple_image

Also, had to change the way we store magic words that are prefixed options. The
MW API sends them back in the form "option=$1", so I store them that way by
default now, and detect them that way in the link handler.

Also, fixed the options like $1px that were hacked in. They all work now.

Change-Id: I41e151e133d63208e0b7e6c1824a612900a3107a