In usage message, mark arguments as optional if there is a default value

spec/usage_spec.lua

@@ -40,6 +40,20 @@ describe("tests related to usage message generation", function()
       )
    end)
 
+   it("creates correct usage message for elements with default value", function()
+      local parser = Parser "foo"
+         :add_help(false)
+      parser:argument "output"
+         :default "a.out"
+      parser:option "--from" "-f"
+         :default "there"
+
+      assert.equal(
+         [=[Usage: foo [--from [<from>]] [<output>]]=],
+         parser:prepare():get_usage()
+      )
+   end)
+
    it("creates correct usage message for commands", function()
       local parser = Parser "foo"
          :add_help(false)

src/argparse.lua

@@ -98,10 +98,16 @@ local Flag = Option:extends {
 function Argument:get_arg_usage(argname)
    argname = self._argname or argname
    local buf = {}
+   local required_argname = argname
+
+   if self._default then
+      required_argname = "[" .. argname .. "]"
+   end
+
    local i = 1
 
    while i <= math.min(self._minargs, 3) do
-      table.insert(buf, argname)
+      table.insert(buf, required_argname)
       i = i+1
    end