Add common interface for formatters

Having a common interface for formatters makes it easier to add
additional formatters. Now to add a new formatter one only needs to
implement the Formatter interface and register the implementation in a
DynamicMap.

Change-Id: I4cbbf6cff89b18a277f8d828486ae9cc023c2dca
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>