Add byTag template function.

Adds a `byTag` template function that accepts a `[]*Service` and returns a `map[string][]*Service` where the map key is the service tag.

An implementation of my suggestion to an alternative solution to #33.
It would be nice to have a little more context in this comment (since it's also the API docs). It's obvious to me that the key (string) is the tag and the value is a slice of Services, but that might not be as apparent to someone just reading the godoc. Can you make this a little more detailed please? :smile: