Imagine usecases, this `config_params` seems not useful.

```
<match **>
  type file
  buffer_symlink_path /path/to/foo
  enable_symlink # or something
</match>
```

I think setter method is better.

```
<match **>
  type file
  symlink_path /path/to/foo # set via @buffer.symlink_path = @symlink_path in configure
</match>
```

What do you think or am I misssing in your proposal?