Allow for arbitrary restrictions enforced by the app

I use this control in an app where the slider values are constrained to match pretty complex rules (to rounded values on an exponential scale) and it's clear that this sort of logic needs to be able to live outside of the slider control, simply by readjusting the slider left and right values in the app itself. Making this work turned out to be a minor change, so here's a pull request of it. I also updated the sample app to demonstrate two such arbitrary restrictions.