Add plugin for SVG images

[jmurty]

Currently the image content plugin does not support SVG files, and there is no way to add an SVG image as a content item to the site.

Adapting the existing image content plugin to handle SVGs properly is probably not the right approach, since there are so many assumptions about images always being raster/pixel-based. We should instead create a new svg_image content plugin just for SVGs.