handling undefined, null, false for data-text & data-value

[matthewmueller]

Right now they will show up literally as undefined, null, & false. Personally I think that undefined and null should probably just be ignored (use the defaults). So in:

<span class="tag" data-text="tag">tag</span>
<input class="url" data-value="url" placeholder="url...">

These values would default back to tag and blank (leading to url... as placeholder).

I think false might be good to either show or not show, but I think that's covered in another attribute.

[tj]

this will get easier with #45