Implement support for 'multiple' attribute and multi-selection

An enhancement to add this functionality, because turns out I needed it :p 

UI behaviour for this mode:
- Dropdown no longer closes when selecting options. Clicking on selected options deselects them.
- Clicking on the top of the dropdown will close it, as will clicking outside of the input on the page.

Essentially, it should work like the normal browser input except obviate the need for the CTRL key.