TaskListItem with TaskStates (instead of a checkbox)

[mikekasprzak]

Inspired by the conversation in #53 and #55 on org-mode, this draft PR is an alternative to #53 that changes the grammar of TaskListItem from containing a checkbox with checked and unchecked states, to containing the status of a task instead: finished, unfinished, and inprogress.

Some notes:

• Feedback is encouraged!
• I initially used done, not_done, and in_progress, but there was another done string in the code I wanted to avoid confusion with, and I didn't want to introduce - or _ into the CSS classes without a conversation first.
• I changed most mentions of checkbox, but some still remain. At the time I couldn't decide if the term checkbox should still be used to describe the [ ] syntax, or if it should be more explicitly called a Task
• The original logic checked for checked or unchecked, then it used an else for the other option. This is handy if you start with checkbox === " " to find the unchecked, as the else will catch both [x] and [X] (and [o]) as checked. I added inprogress into these checks without considering if the logical order should be changed.
• I don't like how they are rendered in HTML (using a CSS class and unicode character), but I didn't have a better idea
• Adding a 4th state like paused ([?]) is easy enough, but the way states are added right now doesn't exactly lend itself to adding more easily (if it's even needed)
• I probably missed something