[DependencyInjection] Added '@@' escaping strategy for YamlFileLoader and YamlDumper

Added the possibility to to use '@@' as an escaping strategy for
parameters that should be treated as strings but start with '@'
(i.e. safe mailer passwords).

I took the '@@' approach from the '%%' escaping strategy and thought
that would fit best, but I leave it open for discussion.