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/new.txt

@@ -0,0 +1,27 @@
+# Test the new command.
+
+hugo new site -h
+stdout 'Create a new site in the provided directory'
+hugo new site mysite
+stdout 'Congratulations! Your new Hugo site is created in'
+cd mysite
+checkfile hugo.toml
+
+hugo new theme -h
+stdout 'Create a new site in the provided directory'
+hugo new theme mytheme
+stdout 'Creating theme'
+cd themes
+cd mytheme
+checkfile theme.toml
+checkfile hugo.toml
+exists layouts/_default/list.html
+exists layouts/_default/single.html
+
+cd $WORK/mysite
+
+hugo new -h
+stdout 'Create a new content file.'
+hugo new posts/my-first-post.md
+checkfile content/posts/my-first-post.md
+