Add support for `display_as` when finding a conversation

[thewheat]

• Current code only allows for finding a conversation without any parameters

      Conversation c = Conversation.find("18718188522");

• Add a new find that allows extra parameters

Sample usage

        Map<String, String> params = new HashMap<String, String>();
        params.put("display_as", "plaintext");
        Conversation c = Conversation.find("18718188522", params);

Docs detailing display_as https://developers.intercom.com/intercom-api-reference/reference#get-a-single-conversation