Major refactor and update. See following:

- Refactored code to decouple listening, selection-counting logic, and
  output view.
- Separated extension state, including browser action icon and title,
  to each tab. Extension state propagates per-tab across page loads.
- Fixed numerous bugs around actual selection tracking. This is tricky,
  and will likely never be perfect (until browsers supply selection
  change events natively).
- Changed selection counting to collapse groups of linebreak whitespace
  to one character.
- Stopped counting newline character at end of line until it is
  actually selected.