Optimisations on init, filtering, dropdown show/hide after profiling 20,000 options.

[joshcaspersz]

I profiled the plugin with 20,000 options and created the following optimisations:
(I've committed each optimisation separately for easy referencing.)

a) Optimised getSelected used in updateButtonText
The jQuery filter was causing a 16 second delay, changing it to the following removed it:
return $(this.$select.get(0).selectedOptions)
selectedOptions is supported in all major browsers except for IE (Edge supports it) so I also implemented a fall back.

b) Optimised buildDropdownOptions
This function was causing the longest delay when loading (nearly 30 seconds).
I've reduced it to under a second by building a string array and then creating the list in one shot.
I disabled dividers and the check for disabled options. You may want to develop further to support these.
Also see XSS fix.

c) Optimised dropdown show/hide
The plugin uses bootstrap drop-downs that toggles the .open style on the dropdown when the button is clicked.
Originally this toggled from display: none to display: block.
Doing so causes a reflow which can take 3 seconds to open and 15 seconds to close on 20000 options.
Toggling visibility:hidden and height:0 instead reduces the delays to about 380ms.

d) Optimised search
This was similar to c), the plugin set display:none or display:block on each list element when filtering.
I removed this and toggled height instead.
This reduced the delay from 1.4 minutes on one search to 800ms.

[dometto]

@davidstutz any chance this could be merged?