Create -tag, which creates tags that apply to all services

This pull request adds the `-tag` command line flag, which causes the specified tags to be applied to all registered services 
The default value of for this is 'registrator', which will tag all services with 'registrator'.
I think it's majorly intrusive to by default add `registrator` to all services. The default should be nothing. And semantically, you can add multiple tags, so this should probably be called `forceTags` and `-tags`