Added support for jshint extends option

Added the extends options according to: https://github.com/jshint/jshint/pull/1314
Implemented in the cli.js of the command line version of jshint in loadConfig on line 532:

      if (config['extends']) {
        var baseConfig = exports.loadConfig(path.resolve(config.dirname, config['extends']));
        config.globals = _.extend({}, baseConfig.globals, config.globals);
        _.defaults(config, baseConfig);
        delete config['extends'];
      }