custom bundle result types

Currently, only html is a valid bundle result output. Support the ability to provide custom functions for generating the `bundle.result.json`. The new api for `bundle.result` should look something like:

``` js
    .pipe(bundle.results({
      dest: './',
      pathPrefix: '/public/',
      outputType: 'html' // or default, or ... Should be a string or function
    }))
```
