specifyBasePath: specify custom basePath

[Remwrath]

Adding the ability to specify a custom basePath in case you would like to use something other than https://gw.cmtelecom.com ie https://gw.cmtelecom.co.za

Usage:

const messagingApi = require("@cmdotcom/text-sdk");

// Get your product token at CM.com.
const yourProductToken = "";
const customBasePath = "https://gw.cmtelecom.co.za";
const myMessageApi = new messagingApi.MessageApiClient(yourProductToken, customBasePath);