Add Flow.js driver

[nerg4l]

Blocked by #43

https://github.com/flowjs/flow.js#handling-get-or-test-requests

• If this request returns a 200, 201 or 202 HTTP code, the chunks is assumed to have been completed.
• If request returns a permanent error status, upload is stopped.
• If request returns anything else, the chunk will be uploaded in the standard fashion.

Closes #9