Allow custom HTTP headers on file upload

This is a feature we needed for a project I am working on in order to integrate with an existing authentication scheme.  The change is relatively simple, so I hope you do not object to the fact that I didn't create a ticket first.

Let me know if I should change anything about the structure/formatting of the change.  Would you prefer me to update the readme file with this new feature?

Also, one suggestion: your contributing documentation is great, except it doesn't mention the component dependency.  It would be helpful to include it along with the directive to run `npm install`.  Let me know if you would like me include that update with this pull request.