Add feature to preprocess chunks prior to testing & sending.

[srijs]

Add a feature that allows for preprocessing each chunk before making requests to the server. This is useful e.g. for client-side checksumming, encrypting or similar processing.

As for your interest in my work, we're currently looking into integrating resumable.js in the next version of the doctape.com file service. Of course we are interested to contribute back as much as possible ;)

[steffentchr]

Awesome feature, much appreciated. Merging.

If you find the opportunity to add a good example of how this might used to the docs, that would be a big help.