Support multiple tags on middleware

[wizbit]

Currently, this bundle does not allow adding multiple tags to a middleware service.

This is required when the same middleware is used on multiple clients (but not all), and when the same service has multiple middleware (e.g. https://github.com/Sainsburys/guzzle-oauth2-plugin)

This pull request allows multiple tags.

[kobelobster]

@cjost1988 we should also add that feature on to our README.md so people know that they are able to use multiple tags. We also should add notes to the UPGRADE.md notifying people of this feature.