add enforce option and pre-commit hook

[drmorr0]

This PR adds an -enforce option to go-carpet that makes the executable return 1 if any file has coverage under the specified -mincov value. It also adds a .pre-commit-hooks.yaml file and a .pre-commit-runner.sh which lets users perform coverage checks as a part of pre-commit.

There's a funny quirk with how pre-commit passes arguments to executables: it just specifies a list of (space-separated) arguments, followed by a list of (space-separated) file names to check. If we want users to be able to customize the -mincov value (or pass any other arguments in) for go-carpet, we need to have some way to differentiate between an "argument" and a "filename".

The best I've been able to come up with is to check for arguments that start with a - character. These are treated as arguments to go-carpet, and everything else gets concatenated together as a list of files.

Please let me know what you think, or if you have any suggestions/concerns!