Add pre-commit config and fix linting

[bplotnick]

This adds pre-commit to run some menial linting tasks.

The way this works is that you just run:

pip install pre-commit  # Just do this once on your machine
pre-commit install  # Do this after cloning for each repo

And then when you commit files, it will run the pre-commit checks against the changed files.

I went ahead and ran these hooks against all of the files and fixed the issues, so there should be no false positives going forward.

[cxuu]

Could you also update the circle config to use precommit as well, so developers would have a consistent experience everywhere?

[jonnylangefeld]

Your machine added this line behind the scenes, because you have go 1.13 installed. However if we merge it, we hav a go.mod with go 1.13, a .circleci/config.yml with go 1.12 and a Dockerfile with go 1.11.
I recommend to remove this line from your PR to not conflict with #26, and then after #26 is merged, everything is up to date on the same version 😊