Always add Attribute xmlns to SVG nodes

[johannesvollmer]

The Svg package for rendering elements automatically adds the namespace.
See https://github.com/elm/svg/blob/dfe97e8282f283f4a62b6645f633076cfd24f3f7/src/Svg.elm#L101

This package will be missing the svg namespace attribute when converted to String. I propose we always add it to any svg root node.

Maybe like this?

{-| -}
svg : List (Attribute msg) -> List (Svg msg) -> Html msg
svg attributes =
    htmlNode "svg" (xmlns svgNamespace :: attributes)

Or maybe like

{-| -}
svg : String -> String -> List (Attribute msg) -> List (Svg msg) -> Html msg
svg namespace version attributes =
    htmlNode "svg" (Attribute.xmlns namespace :: Attribute.version version :: attributes)

Alternatively, in the toString function.