[hash] doesn't work in output.path

re: the documentation here, I expected to be able to use the content hash as a directory:

http://webpack.github.io/docs/configuration.html#output-path

``` js
output: {
  path: path.join(__dirname, 'build', '[hash]'),
  filename: 'build.js'
}
```

yields:

```
build
└── [hash]
    └── build.js
```
