Add support for custom tags

[kuon]

While I understand the will to abstract SE from HTML, the library already providies Element.attribute to set arbitrary attributes. I think it should support setting the tag name in some way.

Either by an attribute, allowing row, column, el... to be altered. Or with a special node function.

Element.html can be used, but the SE nodes it contains have to be rendered to HTML which is a bit troublesome and I guess not very performant.

[mdgriffith]

My hesitance is that changing the node name can result in unexpected breakage.

I'd also like for people to report when they actually need to change the node name and can't use something like html for whatever reason.

[kuon]

I understand the unexpected breakage argument, but in one app I need to use dd and dt and I'd like to use SE within the dd elements.