Support AWS S3

[james-pre]

@atrauzzi:

Cool library, I've been looking for something like a browser-side flysystem.

Out of curiosity, do you have any plans to implement S3 support (both authenticated via the SDK as well as not)? Moreover, would something like EvaporateJS be helpful for unauthenticated S3 uploads?

@jvilk:

I'd accept a PR with an S3 backend. I don't know anything about EvaporateJS and how it compares to the default JS SDK. BrowserFS's internals have not (yet) been optimized for large files, so multipart uploads may be overkill (at least on fast connections).

jvilk/BrowserFS#228

[james-pre]

CC @mcandeia. We can track development of the backend here.