Add simple custom transformers

[rudiedirkx]

Let's say a Feature Context wants to save objects, not ids. Saving shouldn't be a problem, but loading them into a string is. A custom transformer in that Feature Context can fix that. Save objects, print scalars.

In a Feature Context hook:

VariablesDatabase::addTransformer('bla', function(name, value, step) {
  return $value->id;
});

Transformer methods: add, remove, clear.

Like variables, transformers will be cleared after very feature.