Use the upload api of tinypng's homeage to compress images, so can use it without key.
- 模拟用户上传和下载的行为，来得到压缩图片，突破使用官网api每月500张限制
- 通过hash摘要缓存已压缩的文件，只针对有变化的文件进行压缩，避免了重复劳动，提高效率
图片是影响页面加载速度的重要因素，文本代码通常不会很大，在服务端开启gzip压缩可以有一个比较大的压缩率。
但图片本身就是经过压缩的，所以gzip基本不会有任何作用，所以tinypng值得你拥有！
保守估计，本插件可以压缩掉40% ~ 50%, 而对图片质量影响很小！
相比而言，大家熟知的imagemin能压缩10%就不错了。
$ npm i gulp-tinypng-free
var gulp = require('gulp');
var tinypngFree = require('gulp-tinypng-free');
gulp.task('tinypng', function(cb) {
gulp.src('src/*')
.pipe(tinypngFree({}))
.pipe(gulp.dest('dist'));
});
Returns Stream containing compressed images
Type: Object
Default: {}
Sets options described below from its properties.
Type: String
Default: ./sign.json
If set to a filename, it will compare existing source file md5 signatures against those found in the file's json data. When the signatures match, the file is skipped from being minified again, allowing you to better stay within your API request limits. When an image is minified, the md5 signature is determined from the unminified source image and written to the file at options.sigFile (a suggested location would be somewhere under your source control).
Signatures are based off the unminified source image, so that when the source changes it will be re-minified and re-written to the destination file.
Type: Boolean
Default: false
Force compress images regardless of signature. Value can either be true to force all images, or a glob pattern string to match against the filename(s).
need upload files, so it may be unstable.Recommand to move this to the end of task.
尽量放到任务的最后一步，因为这个过程是要上传图片，再下载图片的，和网络稳定有关
https://github.com/creativeaura/gulp-tinypng
https://github.com/stnvh/gulp-tinypng-compress
https://github.com/paper/gulp-tinypng-nokey