Add `allowedTypes` and `resizableTypes` to `imageProcessing` configuration
Prior to this commit, SVG images were not processed by `render-image.html` because SVG does not have a physical dimension like JPEG. This logic was done using a conditional.
I have now realised that Hugo can be very slow when resizing `gif` images. So I created this whitelist mechanism:
- `allowedTypes`: image types with width and height attributes
- `resizableTypes`: image types that can be resized
Here's a list of media types: bmp, gif, jpeg, png, svg+xml, tiff, webp
https://gohugo.io/templates/output-formats/#media-types
