Add autoupdate command to `pre-commit`

Autoupdate will:

```
for each repo in the pre-commit config:
    - fetch inside the repository
    - check the manifest at the tip of `master` to make sure all
        of the configured hooks still exist
    - if any of the hooks no  longer exist: abort with a useful
        error message
    - otherwise update pre-commit config for this repo to point
        at the tip of `master`
```
