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
add tests for bool value setting without default values
Merge pull request #402 from tagomoris/alias_config_attributes

Add 'alias' option to config_section and config_param