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

Author: bep

.gitignore

@@ -1,2 +1,3 @@
 
-*.test
+*.test
+imports.*

cache/docs.go

@@ -0,0 +1,2 @@
+// Package cache contains the differenct cache implementations.
+package cache