Issue 80: mario bros sound when bookmarking a page

I opted for the coin sound, since that seemed to fit more than the
1UP. Open to other suggestions though.

The audio-content.js script might be useful for other Whimsifications
down the line.
When a single-use audio clip has finished playing, delete it.
Switch to a single Mario pref and feature.
Remove invincible.mp3 from this first Mario feature.
Merge pull request #82 from mikeconley/mario

Issue 80: mario bros sound when bookmarking a page