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/options_spec.lua

@@ -258,7 +258,8 @@ describe("tests related to options", function()
       it("handles lack of required argument correctly", function()
          local parser = Parser()
          parser:option "-s" "--server"
-         assert.has_error(function() parser:parse{"--server"} end, "too few arguments")
+         assert.has_error(function() parser:parse{"--server"} end, "option '--server' requires an argument")
+         assert.has_error(function() parser:parse{"-s"} end, "option '-s' requires an argument")
       end)
 
       it("handles unknown options correctly", function()
@@ -289,7 +290,7 @@ describe("tests related to options", function()
             count = 1,
             overwrite = false
          }
-         assert.has_error(function() parser:parse{"-qq"} end, "option '-q' must be used at most 1 time")
+         assert.has_error(function() parser:parse{"-qq"} end, "option '-q' must be used 1 time")
       end)
 
       it("handles too few invocations correctly", function()