Add pre-commit config and flake8 config

[consideRatio]

I'd like to adopt a pre-commit config similar to many sections across the jupyterhub org, with prettier and black being the most relevant hooks added for autoformatting of markdown, yaml, and python files.

I'd like for this to not be merged before #44 or #46, but instead fix the merge conflicts in this PR.

# a command to re-commit the autoformatting stuff
git reset --hard HEAD~5; pre-commit run -a black; git commit -am "pre-commit: run black"; pre-commit run -a prettier; git commit -am "pre-commit: run prettier"; pre-commit run -a end-of-file-fixer; git commit -am "pre-commit: run end-of-file-fixer"; pre-commit run -a requirements-txt-fixer; git commit -am "pre-commit: run requirements-txt-fixer"; pre-commit run -a reorder-python-imports; git commit -am "pre-commit: run reorder-python-imports"

[yuvipanda]

Thanks a lot, @consideRatio