Fetch: Support fetch options

[dr-vortex]

It would be very useful to have the ability to set the options used by the fetch call in the fetchFileAsync function. In particular, this would allow the Fetch backend to access resources that require the authentication header to be set, such as with AWS S3.

I can investigate a PR for this, but do you have thoughts on the best approach to the API. I'm not certain where these options should be exposed in the Configure object. My initial idea would be to add a property to the Fetch options that would be passed down to the fetch request.

~ @halmos

jvilk/BrowserFS#289

[dr-vortex]

Transferring to fetch