Add attribute to strip all HTML from element

I'm using `contenteditable` as a kind of glorified input box. I've found that it's often much easier and flexible to make a `li` or a `h1` editable and style it a little to hint editability than it is to insert regular input boxes. However in these cases I don't actually want the user the ability to insert HTML into the editable.

To support that use case this tiny patch adds a `strip-html` attribute that strips all HTML from the content of the editable element.