Support for Added Methods in API v1.3 #155

Adding this here to capture for a PR. Ideally this SDK will be updated to support the latest API version (currently 1.3) as there are some added endpoints - actually, even some added in v1.2 are not yet included.

Here's one example from 1.2+ for ListLeadsByPhone
https://developers.intercom.com/intercom-api-reference/reference#section-list-leads-by-phone

The change log is here https://developers.intercom.com/building-apps/docs/api-changelog

Also see there are a handful of breaking changes in v1.3