[FEATURE] Configurable namespaces
[FEATURE] Configurable namespaces

This adds a signal to the ``TemplateParser`` that allows for
registering ViewHelper namespaces.

With I965cb54c3125f80e7a5ae46ede72ee9027ed006e that is used to
register all Flow keys of active packages as ViewHelper
namespaces.

Change-Id: Ie4e40713ec7b2a31464ddd633458d757d55d52e7
Resolves: FLOW-151
Releases: master
Original-Commit-Hash: 77e94925fcd546c6f072b06ca1bc5a841366d637

neos/fluid,neos/fluid