Support listing the URLs in a post.

Support listing the URLs in a post.

I didn't have to trawl RcDom after all (as my TODO item suggested).
All I had to do was to put a mutable reference to a Vec<String> into
OurDecorator, and then that could push URLs on to the vector.

Or rather: firstly I had to do that using a &RefCell<Vec<String>>,
because the same Vec<String> reference had to be passed to the
separate decorator instance spawned for sub-blocks. Secondly that
meant I had to bake a lifetime into the type of OurDecorator (since
the call site must know that the reference to the RefCell went away
when the config object is dropped).

Also, I wanted to exclude the URLs that come from mentions and
hashtags, which meant tracking whether a colour annotation had been
applied to anything inside the link - whether it was already live at
the call to decorate_link_start or was turned on immediately
afterwards.