Add 'concat' action

spec/actions_spec.lua

@@ -100,9 +100,10 @@ describe("actions", function()
       local parser = Parser()
       parser:flag("--no-foo"):target("foo"):action("store_false")
       parser:flag("--no-bar"):target("bar"):action("store_false")
+      parser:option("--things"):args("+"):count("*"):action("concat")
 
-      local args = parser:parse{"--no-foo"}
-      assert.same({foo = false}, args)
+      local args = parser:parse{"--things", "a", "b", "--no-foo", "--things", "c", "d"}
+      assert.same({foo = false, things = {"a", "b", "c", "d"}}, args)
    end)
 
    it("for options allow setting initial stored value", function()

src/argparse.lua

@@ -375,6 +375,16 @@ function actions.append(result, target, argument, overwrite)
    end
 end
 
+function actions.concat(result, target, arguments, overwrite)
+   if overwrite then
+      error("'concat' action can't handle too many invocations")
+   end
+
+   for _, argument in ipairs(arguments) do
+      table.insert(result[target], argument)
+   end
+end
+
 function Argument:_get_action()
    local action, init