Find closest .firebaserc up a directory tree

It would be great if the logic for loading the `.firebaserc` looked up the directory tree similar to [read-pkg-up](https://github.com/sindresorhus/read-pkg-up).

This would save having to copy `.firebaserc` files across service folders in monorepo architectures.

currently:
```markdown
- api/
| - .firebaserc
| - firebase.json
- app/
| - .firebaserc
| - firebase.json
- etc...
```

what this would allow:
```markdown
- .firebaserc
- api/
| - firebase.json
- app/
| - firebase.json
- etc...
```

Related code:
- https://github.com/firebase/firebase-tools/blob/78f11b62ab658a5a59b291daad7ea5ee08b2bc34/src/command.js#L159
- https://github.com/firebase/firebase-tools/blob/78f11b62ab658a5a59b291daad7ea5ee08b2bc34/src/rc.js#L191