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

markup/converter/converter.go

@@ -30,15 +30,17 @@ import (
 
 // ProviderConfig configures a new Provider.
 type ProviderConfig struct {
-	MarkupConfig markup_config.Config
-
-	Cfg       config.Provider // Site config
+	Conf      config.AllProvider // Site config
 	ContentFs afero.Fs
 	Logger    loggers.Logger
 	Exec      *hexec.Exec
 	highlight.Highlighter
 }
 
+func (p ProviderConfig) MarkupConfig() markup_config.Config {
+	return p.Conf.GetConfigSection("markup").(markup_config.Config)
+}
+
 // ProviderProvider creates converter providers.
 type ProviderProvider interface {
 	New(cfg ProviderConfig) (Provider, error)