text: Title: Feature request: Streaming
  Body: As per [my comment in #11](https://github.com/jmcnamara/XlsxWriter/issues/11#issuecomment-479149512), it is possible, if not completely straightforward, to stream ZIP files on output, for example using [`zipstream`](https://github.com/allanlei/python-zipstream). The current ZIP member must be finished before the next one can be started, however. The current `zipstream` also requires all the members to be listed beforehand, but this is an artifact of the current implementation that shouldn’t be too hard to fix.
  
  Does this mean `xlsxwriter` could work in `constant_memory` mode without creating any temporary files as long as write operations to different worksheets are not interleaved?
  
  I would be willing to implement this if necessary and possible, but I’m not at all familliar with the internal structure of `xlsxwriter`, so some design help would be appreciated.