Add ability to specify limits manually - and deal with null data values better#30

This is a small change adding the ability to specify the limits to use in the options, rather than having them calculated directly from the data. I've simply added an if statement to choose whether to do the limits calculation (if the limits weren't passed), and changed a few other bits to refer to the right limits values. I've also done a small change to the styling logic. Previously it used `if(!isNaN(featureValue))` but I had problems with this as data loaded from a GeoJSON doesn't tend to have `NaN` values in it, but instead has `null` values. It turns out that just `if(featureValue)` will exclude NaN, null, and undefined - which is probably the best option. I should note here that I'm not an experienced Javascript programmer - so please let me know if things need improving or changing.