* For arguments: "argument 'foo' is required" -> "missing argument 'foo'"
* For options: "option '--foo' must be used at least 1 time" ->
"missing option '--foo'"
* 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.
