[img-conversion] Encode and decode should take/produce streams (#15)

Talking to folks here at Mozilla, we would like it to be possible to use this API with the WHATWG streams API. Use case is to be able to upload while encoding, or decode while downloading via fetch etc. 

+@domenic 

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/img-conversion/issues/15