Support for saving animations in mp4 format

Problem: I frequently have to create animations from my `xarray` datasets. Currently the only way I can do that easily (i.e. without manually saving each frame as a separate png figure and then sticking them together with `ffmpeg`) is by using `matplotlib`. However, `matplottlib` is notoriously slow and that can be pretty frustrating. Sometimes I have to wait a full hour for all my frames to save!

I love the simplicity of `hvplot`! And, given that `hvplot` has the flexibility to use faster backends, it would be awesome if I could something like

```
anim = da.hvplot(groupy="time", **kwargs)
hvplot.save(anim, "anim.mp4")
```

However, doing this fails. The closest alternative possible seems to me to be `hvplot.save(anim, "anim.gif")`, but inexplicably the output is still an `html` file. (I also haven't been able to make the `fmt=png` alternative work with me, so I'm not sure what that produces.)

Is there any chance this feature can be added to `hvplot`?

In the mean time, is there anything I can do with `hvplot` to plot animations (relatively) fast and simple?

Thanks!