SVG Support

[doofin]

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

• [half finished ] 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]

I moved everything to this PR, so we can work together on the same branch.

[fdietze]

I don't know why this PR was closed. And for some strange reason I am not able to reopen it. @doofin is it possible for you to reopen this PR?

[doofin]

I have no permission to edit ,I guess you have to add me to the collaborator list? It seems that you have deleted the svg branch so this is also automatically closed..So you might have to recreate the svg branch (I am also not familiar with github workflow,also my company uses git for collaboration,but the work flow is merge based rather than pull based)

[doofin]

I have add you to the collaborator list of my repo: https://github.com/doofin/scala-dom-types , we can work on that temporarily if you like

[raquo]

@doofin I've just added you as a collaborator to this project, you can probably recreate this branch after accepting the invite

[doofin]

@raquo I haven't receive any invitation and git still complains permission denied when I tried to push,maybe try again?

[raquo]

@doofin you should be able to see your invite here: https://github.com/raquo/scala-dom-types/invitations

[doofin]

@raquo thanks! It works