Using a ReadStream as input

**Is your feature request related to a problem? Please describe.**
I want to be able to cut clips from one or more large videos and concat them (something like a highlight video). To do so I have to copy the whole video to FS and then run ffmpeg on it. I've implemented a version of this nodejs (not a wasm version) and wanted to move towards client side editing. There you could create a readstream and use that as input for ffmpeg. Unfortunately it seems this is not possible in ffmpeg wasm (unless I'm missing something).

**Describe the solution you'd like**
I'd like some way to use a readstream as input for ffmpeg.

**Describe alternatives you've considered**
I'd have to go back to my old implementation or accept load 2Gb+ into memory.