code: function _transform( xslStr, xmlDoc, xsltParams ) {
      const params = xsltParams || {};
      return new Promise( ( resolve, reject ) => {
          libxslt.parse( xslStr, ( error, stylesheet ) => {
              if ( error ) {
                  reject( error );
              } else {
                  stylesheet.apply( xmlDoc, params, ( error, result ) => {
                      if ( error ) {
                          reject( error );
                      } else {
                          resolve( result );
                      }
                  } );
              }
          } );
      } );
  }
docs: Performs a generic XSLT transformation
  
  @param  {[type]} xslDoc libxmljs object of XSL stylesheet
  @param  {[type]} xmlDoc libxmljs object of XML document
  @return {Promise}       libxmljs result document object