Make L.DivIcon accept HTML Elements in options.html

While writing plugin code, I would like to do transformations on the content of a `L.DivIcon`. This can be done through CSS, but for I simple plugin, I don't like the need of including an additional file.

This PR changes `L.DivIcon` to accept `Element`s created with `L.DomUtil.create(...)` besides the normal html string.