[FrameworkBundle] [Console] added format parameter to ContainerDebugCommand

Bug fix: [no]
Feature addition: [yes]
Backwards compatibility break: [no]
Symfony2 tests pass: [yes]

To make the output from the `ContainerDebugCommand` easier to parse, this
patch adds a `format` parameter to the command which defaults to
`plain` (dumps the services like before). An additional `json` format
is implemented to dump the container info in json.