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/language_content_dir_test.go

@@ -315,7 +315,7 @@ Content.
 	nnSect := nnSite.getPage(page.KindSection, "sect")
 	c.Assert(nnSect, qt.Not(qt.IsNil))
 	c.Assert(len(nnSect.Pages()), qt.Equals, 12)
-	nnHome := nnSite.Info.Home()
+	nnHome := nnSite.Home()
 	c.Assert(nnHome.RelPermalink(), qt.Equals, "/nn/")
 }