WP-API JS Client: Add a helper to get model or collection by route

[adamsilverstein]

In the process of adding page/custom post type support to Gutenberg, the need arose to identify the models based on their route - the code passes the route from PHP to JavaScript, and the JavaScript needs to know which model to use. See ​this pull requst for additional details.

We should add helpers to the client to easily find a model or collection based on its route, something like getModelByRoute and getCollectionByRoute - each would search thru the constructed models/collections and return the matching object, or false if none was found. We should also add unit tests to verify the functions work as expected.