Demo was incorrectly named as showing 'Highlighter', fixed to 'Selection'

Closes #1133

The off pun is rather awful, but it does have precedent, and is
more succinct.

This allows multiple handlers to listen for a single event, and makes
the API a bit more uniform.

Issue #725

Add Match Highlighter add-on to lib/utils, a new demo showing it off, and modify index.html to link to the demo