Add option to populate subdocs

Via [this comment](https://github.com/JKHeadley/rest-hapi/issues/102#issuecomment-382386363):

> It would be great if there were some way to specify an embed of 'subdoc.pets' that made populateEmbeddedDocs call .populate({ path: 'subdoc.pets' }) instead of .populate({ path: 'subdoc', populate: { path: 'pets' } }).

Need to develop syntax to specify between the two, ex: `subdoc.pets` vs `subdoc:pets`

Locate populateEmbeddedDocs and trace how embed paths are parsed and passed to populate. Decide how the proposed dot-versus-colon syntax selects the two population forms, then verify that both subdoc.pets and subdoc:pets produce the intended behavior.

Written by the indexing model from the issue text.