add 'disabled' option

For a current project, I have a need to disable/reenable the autocomplete. I addressed this the easiest way I could think of, by adding an option and short-circuiting the `keydown` if it was set. It looks like the option can be modified via `$('input').data('autocompleter').options.disabled = true`.

I didn't compile the minified file because I wasn't 100% sure you'd want to tackle this in the same way (if at all), but that'd be easy to add.