Tips: use as more general percentage calculator.

The guts were already here, just made them all work properly.

- Greatly expand triggering and allow guard to kick back false
  positives
- Use NumberStyler to handle and "make pretty" a variety of number
  formats
- Use named captures to make things easier to follow.

Fixes #690.
Merge pull request #691 from mwmiller/tipsy

Tips: use as more general percentage calculator.