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

hugolib/content_map.go

@@ -129,7 +129,7 @@ type cmInsertKeyBuilder struct {
 }
 
 func (b cmInsertKeyBuilder) ForPage(s string) *cmInsertKeyBuilder {
-	// fmt.Println("ForPage:", s, "baseKey:", b.baseKey, "key:", b.key)
+	//fmt.Println("ForPage:", s, "baseKey:", b.baseKey, "key:", b.key, "tree:", b.tree.Name)
 	baseKey := b.baseKey
 	b.baseKey = s