class Thor::Arguments

Check if the peek is included in enum if enum is provided. Otherwise raises an error.
for --no-string-arg, nil
for --string-arg, just return the current value in the pile
Parse string:

def parse_string(name)
  if no_or_skip?(name)
    nil
  else
    value = shift
    validate_enum_value!(name, value, "Expected '%s' to be one of %s; got %s")
    value
  end
end