Do not animate if duration set to zero

When there are a lot of instances on a page, the animations can drag performance down significantly on updates. If a user sets the duration to zero, an update call should just replace the HTML in the span.

This is probably an edge case but we are hoping to use this plugin on a page where there will be 100+ kCounter instances that get updated simultaneously - which locks up the page for several seconds as the DOM is gnashing away.