Support blurhash placeholder

[srmullen]

First off, thank you for this library! It's been very helpful.

It would be nice to support blurhash as a placeholder. For inlining, they are smaller than the base64 encoding, and I think they look a bit nicer than the currently offered blur option.

I have a pull request just about ready for this if interested. Let me know your thoughts. Thanks!

[matyunya]

If that's the case then we should probably replace current blur with it?

[srmullen]

I think that might be reasonable, but I didn't want to change how things work for those currently using the blur option. Blurhash requires a canvas to render, so maybe it would be better to let users opt into it.

[matyunya]

Canvas would require more CPU to render so that sounds right.