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

publisher/htmlElementsCollector_test.go

@@ -22,7 +22,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/config/testconfig"
 	"github.com/gohugoio/hugo/media"
 	"github.com/gohugoio/hugo/minifiers"
 	"github.com/gohugoio/hugo/output"
@@ -141,9 +141,8 @@ func TestClassCollector(t *testing.T) {
 					if skipMinifyTest[test.name] {
 						c.Skip("skip minify test")
 					}
-					v := config.NewWithTestDefaults()
-					m, _ := minifiers.New(media.DefaultTypes, output.DefaultFormats, v)
-					m.Minify(media.HTMLType, w, strings.NewReader(test.html))
+					m, _ := minifiers.New(media.DefaultTypes, output.DefaultFormats, testconfig.GetTestConfig(nil, nil))
+					m.Minify(media.Builtin.HTMLType, w, strings.NewReader(test.html))
 
 				} else {
 					var buff bytes.Buffer

publisher/publisher.go

@@ -81,7 +81,7 @@ func NewDestinationPublisher(rs *resources.Spec, outputFormats output.Formats, m
 	fs := rs.BaseFs.PublishFs
 	cfg := rs.Cfg
 	var classCollector *htmlElementsCollector
-	if rs.BuildConfig.WriteStats {
+	if rs.BuildConfig().WriteStats {
 		classCollector = newHTMLElementsCollector()
 	}
 	pub = DestinationPublisher{fs: fs, htmlElementsCollector: classCollector}