Add pre-commit metadata (#343)

This allows using FawltyDeps in a pre-commit hook in your project, by
adding something like this to your project's `.pre-commit-config.yaml`:

```yaml
repos:
  - repo: https://github.com/tweag/FawltyDeps
    rev: v0.13.0
    hooks:
      - id: check
        args:
          - --detailed
```

See https://pre-commit.com for more information.

.pre-commit-hooks.yaml

@@ -0,0 +1,6 @@
+-   id: check
+    name: FawltyDeps
+    description: Python dependency checker
+    entry: fawltydeps
+    language: python
+    pass_filenames: false