Mechanism for keeping track of changes to Starlette

[simonw]

This plugin will mainly be an extracted duplicate of https://github.com/encode/starlette/blob/master/starlette/middleware/gzip.py - modified slightly to remove the dependency on the Starlette Headers and MutableHeaders classes.

If Starlette fix bugs or make improvements, I'd like to reflect them here too.

So I'm going to use a variant of Git scraping to track the Starlette implementation and open an issue in this repository any time it changes.