Fix rendering of double-spaced task lists

Task lists which had one or more spaces between their elements were
not rendering correctly because the markdown filter was translating
this into a <p> tag inside the task <li>, which our task list filter
was not expecting.

Update the task list filter to handle this case.
Fix test_filter_rich_task

Subtle rending change (an added space in this case) in our latest
dependencies broke this test... fixed!
Merge pull request #31 from dmarcotte/spaced-task-lists

Fix double-spaced task lists