[Bug] markdownIt的this指向问题

[jinwei1116]

🐛 Bug Report

https://github.com/hinesboy/mavonEditor/blob/master/src/lib/mixins/markdown.js
这个文件中代码有问题
$_render(src, func, res) {
var $vm = this;
var deal = 0;
for (var i = 0; i < needLangs.length; i++) {
var url = $vm.p_external_link.hljs_lang(needLangs[i]);
loadScript(url, function () {
deal = deal + 1;
if (deal === needLangs.length) {
res = this.markdownIt.render(src);
func(res);
}
})
}
}
这个函数中this.markdownIt这个的this指向有问题，这个this应该改文$vm，目前的this是指向不到markdownIt的，会报错

Steps To Reproduce

Screenshots

The expected behaviour

Possible solution (optional, but very helpful)

[thwblueboy]

有人修，但是感觉没人merge和release的样子……
#899