Related to #498.

This will probably not result in the output intending by the author, but
the syntax would be incorrect so the author needs to edit the document
anyway. We just need to ensure the parser does not crash here. Fixes #498.

Fixes #389. Thanks for the report @lazka.

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.

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.

Not sure how I missed that when committing ea4af0d. Also as a side-effect,
this fixes #230 - which brought my previous oversight to my attention.
Thanks for the report @divisoryang.

Also added some tests - including tests of list items without attr_lists.
Sometimes I forget to test markup that does not use an extension when an
extension is enabled. That's what resulted in #230 being reported.