Hooks are too complicated for simple tasks

Hooks are too complicated for simple tasks. Let's say you want to run `task get gen -f` in a directory created by Mason post-gen, and display the output, the most straightforward solution is:

```dart
import 'dart:io';

import 'package:mason/mason.dart';

void run(HookContext context) async {
final workingDirectory = "${Directory.current.path}/packages/{{package}}";

final process = await Process.start(
'task',
['get', 'gen', '-f'],
workingDirectory: workingDirectory,
);

await process.stdout.transform(utf8.decoder).forEach(print);
}
```

Ideally there would be a `hooks` field in brick.yaml, which might look like this

```yaml
name: package
description: Create a package in the /packages directory
version: 0.1.0+1
vars:
package:
type: string
description: Package name
default: my_package
prompt: What do you want to call your package?

hooks:
post-gen: cd packages/{{package}} && task get gen -f
```