Adding compatibility with symfony/expression-language

Adding the ability to process string expressions using the
symfony/expression-language. It keeps backward compatibility to the
provided operators and with any new operator defined using the default
mechanism.

One upside down with this approach is that you can have a full object
inside the context and evaluate on its attributes using a symfony
expression.
Please inject the ExpressionLanguage instance here, it has optional constructor arguments that you can't use now