Add minimum input feature

Adds a “minimum input” feature similar to Select2. You can give it a try on the [demo page](https://rawgit.com/SonicHedgehog/ui-select/minimum-input-demo/examples/demo-minimum-input.html).

I have taken inspiration from both #326 and #336. In contrast, I’ve tried to keep the core changes to a minimum and implemented the feature for all three themes.