Support for application/octet-stream uploads and file chunking

This looks like a great addon. I like the approach of using a service a lot. For me to use this in my project I would need two things. First, I need to support sending uploads without `multipart/form-data` encoding. Basically I would just put the blob content in the request body and use `application/octet-stream` as the content type.

Would you be interested in a PR to add this as an option? Maybe `options.encoding` with options `form` (default) or `octet`?

Second, I'd like to see an option to upload large files using chunking. One way to achieve this would be to depend on something like [resumable.js](http://www.resumablejs.com/). Is this a direction you are interested in?

Please let me know your thoughts. Thanks!