Use pre-commit#347

git pre-commit hooks can catch linting issues on developer PCs before they're ever committed to the repo. [pre-commit](https://pre-commit.com/) is a widely-used tool that allows projects to specify all of the linters they want to run before each commit. This PR introduces a pre-commit config that will enforce standard text linting; black, isort, and editorconfig linting; and idiomatic Python 3.6+ syntax. * Developers with the pre-commit tool installed can simply run `pre-commit install` in the dotbot repo, and from that point forward git will run the tool prior to each commit so the files are linted and auto-fixed. * The dotbot repo can be registered with the [pre-commit.ci](https://pre-commit.ci/) service so that it auto-checks -- and, by default, autofixes! -- incoming PRs. It will also open PRs to keep the pre-commit hook versions updated on a regular basis. _I STRONGLY recommend doing this!_