Download progress for WASM

**Is your feature request related to a problem? Please describe.**
I would like to display a loading of FFmpeg in my website, as a progress bar.

**Describe the solution you'd like**
A function that defines a callback for the download progress

**Describe alternatives you've considered**
Not implementing the feature at all, and polling `ffmpeg.isLoaded()`

**Additional context**
`fetch` can't be used for this, only `XMLHttpRequest`.