Provide API for customization

provide API for customization

```javascript
var assert = require('power-assert').customize({
    output: {
        maxDepth: 2
    }
});
```

TODO
----------
- [x] Spike
- [x] Rough implementation and test
- [x] Refactor
- [x] README
- [x] Update empower 
- [x] Update power-assert-formatter