Add property label to improve application binding

[btopro]

There's currently no way to update the label on the element without destroying the contents of the element or creating a new "label" element outside of the toggle. By providing a label attribute you would be able to listen to change events on the toggle and then update the label to match the new state. Would also fix #106 or at least provide a way of supporting it.

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[btopro]

I signed it!

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