Add pre-commit tool to repository, and run on all files

[dan98765]

Add pre-commit https://pre-commit.com/ and a config with a minimal set of hooks. Run on all files with tox -e pre-commit (also gets run as part of tox). I added a couple pep8 rules to the ignore list that were causing some not-pretty formatting and I didn't want to do any manual restyles as part of this PR. That said, I'm happy to change the config or ignored rules however the maintainers prefer.

Similar PR for graphql-core: graphql-python/graphql-core#178

[syrusakbary]

Thanks for the PR @dan98765!
Merging :)

[dan98765]

Thanks @syrusakbary!
graphql-python/graphql-core#178 is the exact same change but for graphql-core.