Support embedding of other SVGs (e.g. logo)

[dannyadair]

Allow images to be .svg and embed.

Replace placholder with "the inside of the <svg>", should not have CSS otherwise we'd need to merge/rename classes etc.
Also should not have its own viewbox, i.e. all placement/transforming is done by the container.

This way nice clean standalone SVG images like a logo or common design element can sit in a separate file, and the page template just becomes a simple arrangement of text variables and external images.

[dannyadair]

This will also come in handy for custom processing, when you create e.g. a chart in SVG with another library.