Svg Support

[fdietze]

I was able to port the svg-tags from scalatags via vim-macros!

For reference:

• https://developer.mozilla.org/en-US/docs/Web/SVG
• SVG-Tags in scalatags with MDN documentation:
  https://github.com/lihaoyi/scalatags/blob/44cbd0602d40186d33d39b8d4a259f681d2e4a3b/scalatags/shared/src/main/scala/scalatags/generic/SvgTags.scala
• SVG tag to dom-type association in scalatags
  https://github.com/lihaoyi/scalatags/blob/44cbd0602d40186d33d39b8d4a259f681d2e4a3b/scalatags/js/src/main/scala/scalatags/jsdom/SvgTags.scala

• Create generic svg/Attrs.scala trait with typed listings of SVG attributes
• Same for svg/Tags.scala (might need multiple files similar to how HTML tags are done)
• Add JS-specific type aliases of the above to jsdom/defs/package.scala
• Probably need a new key class: SvgAttr (I don't think we should reuse Attr for that), and a canonical builder for it (see canonical dir). Don't want to accidentally use SVG attrs on HTML elements, and without Track parent tag of properties #13 not sure what else we can do. Not sure about this, and IIRC you're not using these concrete classes in Outwatch, so I guess I'll need to think about this some time later.
• For the Tag class, I think no need to create SvgTag, maybe Tag[SVGElement] would be enough. Not sure.
• Lastly, we'd need to add SVG to the bundle in CompileTest. I think I'm ok allowing naming collisions between SVG and HTML, since most librarries will probably choose to namespace everything svg-related. Things within SVG should probably be free of collisions and follow the same naming guidelines as our HTML stuff
• SVG Namespaces?

fixes #20

[fdietze]

Closing in favor of #23