Add getZoomLevel function

[kurkle]

Changes:

• new getZoomLevel function
• onZoomComplete call after resetZoom
• rename chartState to state for consistency
• utilize valueOrDefault - this could break something, it is not checking for null. @joshkel was there a reason for checking null?

[joshkel]

I don't think checking for null is needed; I was mostly being paranoid / overly defensive.

If I'm following the Chart.js code correctly, null can show up for a scale's min and max (see here), however, even if it did, Math.min and Max.max will ignore it, which seems like the desired behavior.