Add support for breaking <svg> into templates

[cray0000]

Add svg option into templates definition which instructs parser to treat all elements in this template as svg:

<index:>
  <svg>
    <g>
      {{each @points as #point}}
        <view is='point' />
      {{/}}
    </g>
  </svg>

<point: svg>
  <g>
    <circle cx='{{ #point.x }}' cy='{{ #point.y }}' r='{{ #point.r }}' />
    <text x='{{ #point.x }}' y='{{ #point.y }}'>{{ #point.label }}</text>
  </g>

Currently if you try to break <svg> into templates it won't show elements from those templates at all because the information about them being part of <svg> is lost and it creates them via document.createElement() instead of document.createElementNS().

So this pull request fixes it. Though I think it should be possible to get rid of the explicit svg in template declaration if we modify the Element.prototype.appendTo() to figure out on execution time whether the element has the <svg> ancestor or not.

[cray0000]

oh, didn't notice that there is already #4, so this pull request is similar to #5, though it implements another solution proposed by @enjalot in #5 (comment) by marking the whole template as svg.