0.2.1

• Properly validate in CommandMapper::OptionValue#validate when an option,
  who's value is not required, is given true.
• Omit nil arguments from CommandMapper::Command#command_argv if the
  argument is not required.
• Improve validation error message for CommandMapper::Types::Num when
  initialized with a range: value.
• Improve validation error message for CommandMapper::Types::Map and
  CommandMapper::Types::Enum.