Added a clearCache method to clear the render cache

I ran into an issue while using selectize where I wanted to render options based on the value set in a previous selectize (e.g. when a school course is chosen, show the times for that course). Calling refreshOptions wouldn't re-render the options because they were cached. I had to clear the renderCache manually, then the render function would get called next time the list was opened.

If this is expected behavior, I think providing a documented method to do so might be clearer. If this doesn't fit in with the intentions of selectize, feel free to close the PR or post any changes you'd like me to make.