Add a difficulty column to filter+table

I'm not sure that, for filtering, the display of "1.5) medium" looks so good compared to the others; maybe (priority * 2 - 1) would look better? This might need to be updated when https://trello.com/c/4C8w1z5h/17-task-difficulty-settings-v2-priority-multiplier changes.
Add enabled/disabled to Task Overview
Show whether habits are plus or minus, daily days, and todo due dates. The main site
uses an opacity of 0.3 for skipped days and 0.5 for disabled directions; I arbitrarily
chose 0.4 to split the difference. :)
Task Overview HTML boxes
Make the additional task toggles (Habit directions, Daily repeat days) look like boxes; use an en dash instead of a hyphen for minus.
Show To-Dos without Dates below the To-Dos with Dates
The variable names probably aren't ideal. The comparison of items.length with zero I'm guessing is typo'd, but it could maybe be ```items.length + itemsNotDue.length```.
Merge pull request #10 from goldfndr/master

Add a difficulty column to filter+table for Task Overview