Support for dropdownDirection

Thanks for a great plugin! I have been implementing it across our product over the last weeks.

I encountered some problems with dropdowns being clipped when appearing close to the bottom of the screen. I found your comment in one of the issues to implement `dropdownDirection` (https://github.com/brianreavis/selectize.js/issues/29).

Also added support to reduce height of dropdown in case it can't fit in the window (both up and down).