Touchstart for select/deselect event

[helenawin]

Tested on Safari iphone 7, had to double tap items in dropdown box to select/deselect them. Hard-fixed by adding touchstart eventlistener for mobile devices with the same function as it is now only for click. (line 1339 /src/selectr.js)

this.tree.addEventListener("click", function(e) { var item = util.closest(e.target, function(el) { return el && util.hasClass(el, "selectr-option"); });