Support multiple values for tags and opaque tag byte tag values.

[smcallis]

Not all tags naturally have a single value, so for MVT3 it'd be nice to support repeatedly specifying the same tag and different values for it. The client could decide to expose this in a couple of ways. Alternatively repeated fields could be one of the optional tag types.

Also, it'd be nice to have an optional bytes tag value that could hold some application-specific encoding of data (e.g. another protobuffer).