Add KmzFile overloads to use different underlying zip stream

[p-skakun]

This pull request adds the ability to specify a custom underlying stream for the kmz zip archive. In certain scenarios, such as limited resources, handling huge files, or frequent operations, using MemoryStream may not be the best option. Changes can be explicitly flushed by calling the Flush() method if needed.

[samcragg]

Thanks for contributing, code looks fine with me!

I'll get a build done with the changes tomorrow and publish that to NuGet (it's been a while since I've done a release).

Thanks again