Add feature to exclude packages

[vishwarajanand]

I want to exclude "some" packages (but not all dev dependencies) from the generated graph, can I have something like this?

<?php

$analyzer = new \JMS\Composer\DependencyAnalyzer();
// $excludedPackages can contain packages which I dont want, say `phpunit`
$graph = $analyzer->analyze($dir, $excludedPackages);