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

config/services/servicesConfig_test.go

@@ -54,7 +54,7 @@ disableInlineCSS = true
 func TestUseSettingsFromRootIfSet(t *testing.T) {
 	c := qt.New(t)
 
-	cfg := config.NewWithTestDefaults()
+	cfg := config.New()
 	cfg.Set("disqusShortname", "root_short")
 	cfg.Set("googleAnalytics", "ga_root")