improvements #minor

[nmichlo]

1. swap to pyproject.toml
   e.g. pip install doorway[extra]

2. allow iterable to be passed to download_threaded instead of sequence

3. add new default proxy scraper from 'proxylist.geonode.com', no longer requires bs4

4. allow skip_existing in io_download

5. change default AtomicOpen open mode to r instead of x

6. import .gitignore

7. overhaul EnvVar

8. examples in README

9. rename sharded_and_grouped to sharded_weighted

10. fixed kB (KB is technically incorrect)