Add hidden property, unlisting elements from usage and help

spec/help_spec.lua

@@ -182,4 +182,46 @@ Options:
       assert.equal([[
 I don't like your format of help messages]], parser:get_help())
    end)
+
+   it("does not mention hidden arguments, options, and commands", function()
+      local parser = Parser "foo"
+      parser:argument "normal"
+      parser:argument "deprecated"
+         :args "?"
+         :hidden(true)
+      parser:flag "--feature"
+      parser:flag "--misfeature"
+         :hidden(true)
+      parser:command "good"
+      parser:command "okay"
+      parser:command "never-use-this-one"
+         :hidden(true)
+
+      assert.equal([[
+Usage: foo [--feature] [-h] <normal> <command> ...
+
+Arguments:
+   normal
+
+Options:
+   --feature
+   -h, --help            Show this help message and exit.
+
+Commands:
+   good
+   okay]], parser:get_help())
+   end)
+
+   it("omits categories if all elements are hidden", function()
+      local parser = Parser "foo"
+         :add_help(false)
+      parser:argument "deprecated"
+         :args "?"
+         :hidden(true)
+      parser:flag "--misfeature"
+         :hidden(true)
+
+      assert.equal([[
+Usage: foo]], parser:get_help())
+   end)
 end)

spec/usage_spec.lua

@@ -116,6 +116,42 @@ Usage: foo <first> <second-and-third> <second-and-third>
       )
    end)
 
+   it("omits usage for hidden arguments and options", function()
+      local parser = Parser "foo"
+         :add_help(false)
+      parser:flag "-d" "--deprecated"
+         :hidden(true)
+      parser:flag "-n" "--not-deprecated"
+      parser:argument "normal"
+      parser:argument "deprecated"
+         :args "?"
+         :hidden(true)
+
+      assert.equal(
+         [=[Usage: foo [-n] <normal>]=],
+         parser:get_usage()
+      )
+   end)
+
+   it("omits usage for mutexes if all elements are hidden", function()
+      local parser = Parser "foo"
+         :add_help(false)
+      parser:mutex(
+         parser:flag "--misfeature"
+            :hidden(true),
+         parser:flag "--no-misfeature"
+            :action "store_false"
+            :target "misfeature"
+            :hidden(true)
+      )
+      parser:flag "--feature"
+
+      assert.equal(
+         [=[Usage: foo [--feature]]=],
+         parser:get_usage()
+      )
+   end)
+
    it("usage messages for commands are correct after several invocations", function()
       local parser = Parser "foo"
          :add_help(false)