Choose your own digest, and general improvements

New this release

• New -d flag on put operations to choose what digest to use (defaults to SHA256). Thanks to Brett Filiatreault
• Docs improvements from @mbainter, @egOgre, and @NightKev
• flake8 improvements from @jessemyers
• Tests from @vccabral

Thank you everyone for your contributions!

You can download this new release from pypi by doing pip install credstash.