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/gen.txt

@@ -0,0 +1,19 @@
+# Test the gen commands.
+# Note that adding new commands will require updating the NUM_COMMANDS value.
+env NUM_COMMANDS=41
+
+hugo gen -h
+stdout 'A collection of several useful generators\.'
+
+hugo gen doc --dir clidocs
+checkfilecount $NUM_COMMANDS clidocs
+
+hugo gen man -h
+stdout 'up-to-date man pages'
+hugo gen man --dir manpages
+checkfilecount $NUM_COMMANDS manpages
+
+hugo gen chromastyles -h
+stdout 'Generate CSS stylesheet for the Chroma code highlighter'
+hugo gen chromastyles --style monokai
+stdout 'color: #f8f8f2'