* Adding more flexibility to describe a feature

Motivation:

Only adding a stability to describe a feature is often not enough,
instead we now allow to add the following information.

Fullname:
This is a more human readable version of the feature.
Example: Jolokia autodiscovery

Doc:
This is a one liner describing what you can do with the feature.
Example: The dissect processor allows to extract useful parts of the
original string.

Stability:
Describe how stable is the current feature.

* Remove the String() but keep the stringer interface implementation

* s/Describeable/Describer

* Add FilterWith that take a function predicate to filter the feature.

Currently we only provide the HasStabilityPred to filter feature from a
bundle, but this change open the door to chainable filterWith.

Example:

```
bundle.FilterWith(NamespaceMatchPred("processor"))
   .FilterWith(HasStabilityPred(feature.Beta))
 ```

* Update overwrite signature.

(cherry picked from commit 91af9a7)