Make sure we pass in an argument to focusOn directive #10

[guikubivan]

When used in an ng-repeat loop, I only wanted it to trigger focus on first item, so I did something like this:

<a ... focus-on="{{$first ? 'focus-just-on-me' : '' }}>...</a>

However, that made it add listeners even for when focus-on was not defined.

PR at #10 if interested.