Add 'animated' constructor property as shortcut for 'pages'

Provides easier-to-understand API when handling animated images

lib/output.js

@@ -399,6 +399,7 @@ function webp (options) {
  * @returns {Sharp}
  * @throws {Error} Invalid options
  */
+/* istanbul ignore next */
 function gif (options) {
   if (!this.constructor.format.magick.output.buffer) {
     throw new Error('The gif operation requires libvips to have been installed with support for ImageMagick');