[active-line addon] Don't highlight line(s) with a selection.

This fixes non-empty selections having 2 highlights: the selection highlight and the active-line highlight. Non-empty selections should **only** have the selection highlight.

A before-and-after comparison (note the confusing blue active-line highlight on the same line as the purple selection highlight):

Before:
![before](https://cloud.githubusercontent.com/assets/213/3073698/45a9a106-e300-11e3-824b-450300390b7d.gif)

After:
![after](https://cloud.githubusercontent.com/assets/213/3073699/48450126-e300-11e3-8dc8-8beedfaa3ffb.gif)