Use a registry instead of plain PHP array for formatter registration

[DanielSiepmann]

This allows to inject the formatters in other classes. This also eases to replace the feed manager but still receive the formatters, as they are no longer hard injected into a specific PHP class / service but provided via a registry.

Q	A
Branch?	"master"
Bug fix?	no
New feature?	yes
BC breaks?	Gues so, signature of the FeedManager constructor has changed. Not sure if that was considered public API.
Deprecations?	no
Tests pass?	yes/no
Fixed tickets
License	MIT
Doc PR