Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

resources/images/image_resource.go

@@ -62,6 +62,6 @@ type ImageResourceOps interface {
 	// using a simple histogram method.
 	Colors() ([]string, error)
 
-	// Internal
+	// For internal use.
 	DecodeImage() (image.Image, error)
 }