Add 'alias' option to config_section and config_param

 * To allow plugin authors to define alias name of configuration attributes
   * An valid attribute name in standard configuration syntax, MAY conflicts ruby reserved keywords in DSL
   * An valid attribute name in DSL, MAY conflicts in Fluentd configuration syntax keywords
`conf[opts[:alias].to_s]` is a value that user specified. It will cause error if it is missing and `default` option is not specified.