Request to support images from the network

[ouyangjunyi]

trafficstars

Now only local images are supported. However, web images are not supported.

[ouyangjunyi]

![image](https://raw.githubusercontent.com/ouyangjunyi/PicGo/master/images/20201214222127.png)

[jasonwilliams]

The change would be here: https://github.com/jasonwilliams/anki/blob/master/src/markdown/Serializer.ts#L127-L136

Good first issue for someone to pick up