Support for SVG

[dieggsy]

I tried defining an svg using keyword tags within a net.html.generator:html and got an "unknown html keyword" error. I sort of got around this by using the internal:

(net.html.generator::def-std-html :svg t nil)
(net.html.generator::def-std-html :g t nil)
(net.html.generator::def-std-html :text t nil)
(net.html.generator::def-std-html :rect t nil)
;; ... etc.

But it would be nice to support this by default or to make def-std-html external for this purpose.