Feature Request: support multiple messages (or warningMessages) from validators

Since an attribute can have multiple messages from multiple validators, it doesn't seem too far fetched to add support for multiple messages from a single validator. For example, I have a custom password strength validator (based on zxcvbn) that can provide multiple suggestions back to the user to improve password strength. If I try to return multiple messages today it seems to get awkwardly stringified, and it breaks the ability to iterate over the messages in an Ember template.

What I'd _really_ like is the ability to return a collection of results that can include both warning results and non-warning results. The validation would be considered invalid if the collection included any non-warning results. That would be a more significant change based on my reading of the source.