v0.2.0

• Added CommandMapper::Command.spawn and CommandMapper::Command#spawn_command.
• Added checks to CommandMapper::Command.option, CommandMapper::Command.argument, and
  CommandMapper::Command.subcommand to avoid overwriting an existing option/argument/subcommand
  with the same name.
• Added the value_in_flag: keyword argument to
  CommandMapper::Command.option which indicates an option's value
  should be appended to the flag (ex: -Fvalue).
• Added the range: keyword argument to CommandMapper::Types::Num#initialize
  for specifying the acceptable range of numbers.
• Allow options with equals: true (aka --opt=...) or value_in_flag: true
  (aka -Fvalue) to accept values that start with a - character.