Consider adding linting as a git hook before pushing

Having linting as a git hook before pushing will prevent submission of broken PRs for review.
The downside is it will slow down pushing in the common case (user has linted themselves and fixed errors) and it will require a little bit of work in butler.py to get the git hooks working without user intervention.

@oliverchang @inferno-chromium @Dor1s @mbarbella-chromium
What do you think?