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/hugo__watch.txt

@@ -0,0 +1,28 @@
+# Test the hugo command.
+
+# See https://github.com/rogpeppe/go-internal/issues/228
+[windows] skip
+
+hugo -w &
+
+sleep 3
+grep 'P1start' public/p1/index.html
+
+replace content/p1.md 'P1start' 'P1end'
+sleep 2
+grep 'P1end' public/p1/index.html
+
+stop
+
+-- hugo.toml --
+baseURL = "http://example.org/"
+disableKinds = ["RSS", "sitemap", "robotsTXT", "404", "taxonomy", "term"]
+-- layouts/index.html --
+Home.
+-- layouts/_default/single.html --
+Title: {{ .Title }}| {{ .Content }}
+-- content/p1.md --
+---
+title: "P1"
+---
+P1start