Support embedding of other SVGs (e.g. logo) #29

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.