Optional validation

How can one validate a field _only if it's present_?

Currently, If a value is null or undefined, validation [always passes](https://github.com/kripod/bookshelf-validate/blob/master/lib/index.js#L15).

My use case is allowing null values for postcodes, but if one is provided then it must be valid.
So there either must be a valid one, or none at all.

I was hoping to achieve this with a custom validator:

```
postcode: [{
method: 'isPostcode',
error: 'Postcode is invalid',
args: { optional: true }
}]
```

Read lib/index.js around line 15 to trace how null and undefined values currently bypass validation. Define the optional-validation behavior for a missing postcode versus a provided valid or invalid postcode, then verify those three cases against the repository's existing validation behavior.

Written by the indexing model from the issue text.