Feature/allow custom endpoints

[Rainymood]

Summary

This PR:

• Refactors the hardcoded URLs in lib/Transport.js to a convict config
• Adds functionality to overwrite these urls if needed (for example if you use a reverse-proxy)

Usage

If you are behind a proxy and need to use http instead of https you can
overwrite the default endpoints that Chatbase uses as follows:

var chatbase = require('@google/chatbase')
	...
	.setCreateEndpoint('http://custom-create-endpoint.com/api/message'); // default: https://chatbase-area120.appspot.com/api/message
	.setCreateSetEndpoint('http://custom-create-set-endpoint.com/api/messages'); // default: https://chatbase-area120.appspot.com/api/messages
	.setUpdateEndpoint('http://custom-update-endpoint.com/api/messages/update'); // default: https://chatbase-area120.appspot.com/api/message/update

Why

The current URLs are hardcoded in lib/transport.js and there is no easy way of changing them in the current node package, making it difficult to use Chatbase behind a proxy which doesn't allow, for example, https connections.

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