Allow inline attr_lists at end of header.

A header can be only one line - so unlike other block level elements, a
attr_list must be at th eend of the line. To disingush it from an inline
attr_list on the last child in a header, we must require at least one
space before th eblock-level attrt_list.

Always intended it to work that way. Not sure how I missed that before.
At least we're testing for it now.
Added support for attr_lists on definition list terms.

Like headers (h1-6) dt's can only be on one line, so we need to use
the header regex on dt's. This was implemented after considering a recent
[discussion](http://six.pairlist.net/pipermail/markdown-discuss/2013-July/002859.html)
on the markdown mailing list. Prior to that discussion, I never considered
that it was impossable to set block level attrs on dt's. Now it is.