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.

A list item with a nested list complicates were the attr_list can be.

Fixes #218. Thanks for the report @jpzimmer.

Also cleaned up poor wording on other setting for codehilite.

Thanks for the inital suggestion @davidcorne. Fixes #223.

Just set each abreviation as an AtomicString. Given the nature of
abbreviations, they are not likely to ever contain any other markup
anyway. Also added a test.

Fixes #224. Thanks for the report @JakeChampion.