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/alias.go

@@ -101,7 +101,7 @@ func (s *Site) publishDestAlias(allowRoot bool, path, permalink string, outputFo
 		OutputFormat: outputFormat,
 	}
 
-	if s.Info.relativeURLs || s.Info.canonifyURLs {
+	if s.conf.RelativeURLs || s.conf.CanonifyURLs {
 		pd.AbsURLPath = s.absURLPath(targetPath)
 	}