Implement commonmark-hs compatible definition lists

[notriddle]

Fixes #67

[Martin1887]

Awesome! But I think it would be better to merge it in the new branch_0.12 branch I have just created, because the last changes probably deserve a new 0.11 minor release.

Also, I wonder if the number of calls to self.finish_list in firstpass.rs could be reduced by dividing it into a function for normal lists and another one for description lists or something else. It seems the only messier part of the code respect to the previous version.

Thanks!

[notriddle]

Okay, I've rebased onto 0.12.

[notriddle]

For simplicity's sake, I'm going to use the term item lists to refer to numbered and bulleted lists, as opposed to "definition lists."

Also, I wonder if the number of calls to self.finish_list in firstpass.rs could be reduced by dividing it into a function for normal lists and another one for description lists or something else. It seems the only messier part of the code respect to the previous version.

Unfortunately, it's not quite that simple. self.last_line_break and self.begin_list_item are used by both definition lists and item lists. The functions finish_list and finish_empty_list_item both clear these variables out after "consuming" them to mark the list as loose or to pop out of the empty list item.

If there's a cleaner way to handle these cases, then maybe?

[Martin1887]

A lot of time since the last update and I don't get a cleaner manner of doing it, so let's merge it. Thanks!