If selectlist is empty, disable it

[interactivellama]

Fixes #1349 by checking to see if li are present and disabling itself if there are none or only one single item.

What's the point of selectlist with 1 item or no items?

Also implements an empty list label API:

var $selectlist = $(html).find('#selectlistEmpty').selectlist({
    emptyLabelHTML: '<li data-value=""><a href="#">I am feeling so empty</a></li>'
});

This is getting a little imperative with the emptyLabelHTML.

UPDATED: Design prefers to behave as a native select element and still allow the dropdown if only a single item exists.

[jamin-hall]

@interactivellama let's try it this way.

[interactivellama]

How do we want to allow the developer to create "a blank selectlist"? Right now, you can create the state by adding the empty option to the dropdown, initializing it, and then disabling it. Do we want to make this easier?

[kevinparkerson]

We probably want a min-height on the selectlist, to ensure blank selectlists are not super tiny as well