Allow the user to define a baseRoutePattern for child admin

I need to define the route pattern for my child admin.

here an example:

LicenceContractAdmin => /admin/contract/licence

LicenceContractClauseAdmin => /admin/contract/licence/{id}/clause

You just need to define 

```php
<?php
class LicenceContractAdmin extends Admin
{
    protected $baseRoutePattern = "/admin/contract/licence";
}

class LicenceContractClause extends Admin
{
    protected $baseChildRoutePattern = "clause"
}
```

I do the same thing for baseRouteName with baseChildRouteName