Add a hasFinder method to table object

Adds a simple method for checking if a table has a finder available to use.

Usage:
```php
$table->hasFinder('list');
```

Useful for something like
```php
if ($table->hasFinder($customFinder)) {
    $table->find($customFinder);
}
```

Cleaner (imo) than having a no-op catch block for the `BadMethodCallException`
Could you add a doc block here?