Redesign argument storing

* Use state objects instead of tons of locals in the main
  function.
* Use actions for storing arguments into result table.
  Actions are now called at the end of each invocation,
  with result table, target index, arguments and overwrite flag as
  arguments.
* Remove command actions.
* Improve error messages, refer to options by the last used alias
  instead of the main name.

TODO:

* Improve error messages further ("argument 'foo' is required"
  -> "missing argument 'foo'", etc.).
* Add actions for positional arguments.
* Add actions for commands (should be called with final results
  after parsing is over, in "innermost first" order).
* Allow referring to built-in actions by strings a-la Python
  (e.g. action = "store_false").
* Allow setting initial value to be stored at target index
  for each option (perhaps use default value for that).
* Add more tests, particularly for actions.

spec/arguments_spec.lua

@@ -97,7 +97,7 @@ describe("tests related to positional arguments", function()
          local parser = Parser()
          parser:argument "foo"
 
-         assert.has_error(function() parser:parse{} end, "too few arguments")
+         assert.has_error(function() parser:parse{} end, "argument 'foo' is required")
       end)
 
       it("handles extra arguments with several arguments correctly", function()
@@ -113,7 +113,7 @@ describe("tests related to positional arguments", function()
          parser:argument "foo1"
          parser:argument "foo2"
 
-         assert.has_error(function() parser:parse{"bar"} end, "too few arguments")
+         assert.has_error(function() parser:parse{"bar"} end, "argument 'foo2' is required")
       end)
 
       it("handles too few arguments with multi-argument correctly", function()
@@ -121,7 +121,7 @@ describe("tests related to positional arguments", function()
          parser:argument "foo" {
             args = "+"
          }
-         assert.has_error(function() parser:parse{} end, "too few arguments")
+         assert.has_error(function() parser:parse{} end, "argument 'foo' is required")
       end)
 
       it("handles too many arguments with multi-argument correctly", function()
@@ -137,7 +137,7 @@ describe("tests related to positional arguments", function()
          parser:argument "foo" {
             args = "2-4"
          }
-         assert.has_error(function() parser:parse{"foo"} end, "too few arguments")
+         assert.has_error(function() parser:parse{"foo"} end, "argument 'foo' requires at least 2 arguments")
       end)
 
       it("handles too many arguments with several multi-arguments correctly", function()
@@ -159,7 +159,7 @@ describe("tests related to positional arguments", function()
          parser:argument "foo2" {
             args = "*"
          }
-         assert.has_error(function() parser:parse{} end, "too few arguments")
+         assert.has_error(function() parser:parse{} end, "argument 'foo1' is required")
       end)
    end)
 end)