Add feature enhancement

[sveyret]

It would be good to have a way to enhance a feature. For example, if I create a type Date and I want to ensure that an object of type Date is equal to a value, I would like to override the current equal feature to treat my Date object.

We could have, for example:

function replaceFeature(name, replaceFn)

Where name is the name of the replaced feature, and replaceFn is a function taking the previous function as first parameter and returning a new function managing the new behavior.