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/resource_transformers/minifier/minify.go

@@ -30,7 +30,7 @@ type Client struct {
 // New creates a new Client given a specification. Note that it is the media types
 // configured for the site that is used to match files to the correct minifier.
 func New(rs *resources.Spec) (*Client, error) {
-	m, err := minifiers.New(rs.MediaTypes, rs.OutputFormats, rs.Cfg)
+	m, err := minifiers.New(rs.MediaTypes(), rs.OutputFormats(), rs.Cfg)
 	if err != nil {
 		return nil, err
 	}