Support to call php code on any form option

Adds support to call custom php code on any form option.
The code which should be executed has to be wrapped with __php(<<php code>>).

Example usage:
...
addFormOptions:
    languages: __php($this->getLanguages())
    type: __php(new \Foo\Bar\BazType)
    ...
Made selector non-greedy
Merge pull request #339 from kufi/patch-4

Support to call php code on any form option