Core API: find newsletter by email address

Currently the Core API only allows newsletters to be retrieved by id. It would be useful to be able to retrieve newsletters by email address.

Example use case:

A company using Pelcro is starting a email newsletter. The marketing department has a list of email addresses they would like to opt-in to this new newsletter.

Using the Core API, I would like to do the following:

For each email address:

• Look up an existing newsletter object in Pelcro with a matching email address.

• If a newsletter object exists, append a new list id to the lists field and save this to Pelcro (HTTP PUT).

• If a newsletter object does not exist, create a new object and set lists to the new list id (HTTP POST).