CDN support

Another way, you can simply add cdn url like this:

```
# hexo-tag-emojis plugin configuration
emojis:
  cdn: http://cdn.staticfile.org/emojify.js/0.9.0/emojis
```
In this way, nothing will install into you blog, all images are from the CDN server,
This is particularly useful for the blog hosted on github,
because github is really slow for images