support custom separator

With custom separator I can do wildcard searches in strings like `controller:users`. I implemented this passing a third param to wildcard method. Example:

```javascript
var wildcard = require('wildcard');

var registry = {
  'controller:users': true,
  'controller:posts': false
};

wildcard('controller:*', registry, ':');
//=> {  'controller:users': true, 'controller:posts': false }
```