add support for streaming while rendering

This is a working proof of concept for producing a stream for saving or sending (e.g., from a web server) an xlsx document before it has completely rendered the entire document. This is useful for very large datasets where you would like to start the sending the file as soon as possible so that the user can start downloading it. I would like to know if the approach is something you would find acceptable, and if so, what revisions would be needed to have it accepted. Although it is fairly simple, I had to shoehorn the additional functionality into the gem. There may be a better way to achieve the same result, one which would apply not just to streaming the worksheet, but all the other xml documents as well. A limitation of the current code is that I had to disable validation for the worksheet. However, since the additional functionality has a feature flag, this limitation only applies when streaming. All the tests are passing. Any feedback would be appreciated.