Allow Developers to Disable Selectize Autogrow

There are times autogrow isn't wanted and times the current autogrow implementation in Selectize.js causes serious performance issues. We had a page that was taking 40 seconds to Selectize around 1700 select boxes, and traced the hotspot back to the autogrow code in Selectize. With autogrow disabled, we were able to Selectize all of those boxes in around 2 seconds.