Use custom naming scheme for optimized file

Output naming scheme is hardcoded in `BorschikNode` as 

```javascript
// lib/nodes/borschik.js

registry.decl(BorschikNodeName, GeneratedFileNode, {
  // ...
  this.output = PATH.join(PATH.dirname(this.input), '_' + PATH.basename(this.input))
```

Guess it should be customizable.