Parser improvements:
* Flag parameters' short argument form can now be chained correctly.
* FlagParameter's initializer accepts a custom value to replace False
* FlagParameters now accept --flag=0 and similar as negation.
* Added tests for Parameter.LAST_OPTION
* Parameter.read_argument is now called with (bound_arguments, position)
* Its return value is no longer evaluated, so it is expected to change
  attributes of bound_arguments when needed instead.
* Removed all instances of read_one_argument