[RFC] Added `--apply-closure` option

This option gives us a very quick and flexible solution for migrations:

````
./bin/phpcr phpcr:nodes:update --query="SELECT * FROM [nt:unstructured] WHERE ..." --apply-closure="function ($session, $node) { if (null !== $node->getProperty('url')) { $node->setProperty('linkType', 'externalUrl'); }"
```` 
What do people think?