New option + checks for add-parents.

This solve https://github.com/fhaculty/graph-uml/issues/2 and add option to just render given classes by 'add-parents'.

To render only the classes (without adding relations and/or parent class interfaces try this.

```php

$graph = new Fhaculty\Graph\Graph();
$builder = new Fhaculty\Graph\Uml\ClassDiagramBuilder($graph);
$builder->setOption('add-parents', FALSE);

$builder->createVertexClass('Fhaculty\Graph\Edge\Base');
$builder->createVertexClass('Fhaculty\Graph\Edge\Directed');

$graphviz = new Fhaculty\Graph\GraphViz($graph);
$graphviz->setFormat('svg');
$graphviz->display();
```