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/prune_resources.go

@@ -15,5 +15,5 @@ package hugolib
 
 // GC requires a build first and must run on it's own. It is not thread safe.
 func (h *HugoSites) GC() (int, error) {
-	return h.Deps.FileCaches.Prune()
+	return h.Deps.ResourceSpec.FileCaches.Prune()
 }