Allow overriding of element namespace via :ns attr

[gerrit-hntschl]

Hi,

just opened this very basic PR to discuss with you how to add support for SVGs foreignObjects.

If this approach seems workable for you I will add some more documentation.

This PR allows it to create foreignObjects in this way:

[:foreignObject {:ns        "http://www.w3.org/1999/xhtml"}
  [:body
    [:p "Nicely wrapped text"]]]

What do you think?

Cheers,
Gerrit

[gerrit-hntschl]

@pallix ping. would be highly appreciated if we could progress on this!

[pallix]

I am sorry I missed your PR. Are you still interested to discuss it?