Batch get function

[rclark]

Some workflows could benefit from a request where you provide a set of ids and get back all the features. DynamoDB provides a BatchGetItem function that is what we'd want to use, and Dyno provides a simpler wrapper on it.

Implementing this would mean:

• adding a batch.get function to https://github.com/mapbox/cardboard/blob/master/lib/batch.js
• taking inspiration from the single-feature get function on how to resolve features given their ids
• writing tests to prove that it works in https://github.com/mapbox/cardboard/blob/master/test/batch.test.js

cc @peterqliu