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

testscripts/commands/list.txt

@@ -0,0 +1,34 @@
+# Test the hugo list commands.
+
+hugo list drafts
+! stderr .
+stdout 'draft.md,2019-01-01T00:00:00Z'
+
+hugo list future
+stdout 'future.md,2030-01-01T00:00:00Z'
+
+hugo list expired
+stdout 'expired.md,2018-01-01T00:00:00Z'
+
+hugo list all
+stdout 'future.md,2030-01-01T00:00:00Z'
+stdout 'draft.md,2019-01-01T00:00:00Z'
+stdout 'expired.md,2018-01-01T00:00:00Z'
+
+-- hugo.toml --
+baseURL = "https://example.org/"
+disableKinds = ["taxonomy", "term"]
+-- content/draft.md --
+---
+draft: true
+date: 2019-01-01
+---
+-- content/expired.md --
+---
+date: 2018-01-01
+expiryDate: 2019-01-01
+---
+-- content/future.md --
+---
+date: 2030-01-01
+---