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/pparse_spec.lua

@@ -15,13 +15,13 @@ describe("tests related to :pparse()", function()
       parser:argument "foo"
       local ok, errmsg = parser:pparse{}
       assert.is_false(ok)
-      assert.equal("too few arguments", errmsg)
+      assert.equal("argument 'foo' is required", errmsg)
    end)
 
-   it("still raises an error if it is caused by misconfiguration", function()
+   it("rethrows errors from callbacks", function()
       local parser = Parser()
       parser:flag "--foo"
-         :action(error)
-      assert.has_error(function() parser:pparse{"--foo"} end)
+         :action(function() error("some error message") end)
+      assert.error_matches(function() parser:pparse{"--foo"} end, "some error message")
    end)
 end)