Add extension methods to ControllerBase for TwiML

[Swimburger]

This PR adds TwiML extension methods for ControllerBase to create a TwiMLResult.
This allows developers to use the TwiML methods without having to inherit from TwilioController.

A developer would use the extension method like this:

using Twilio.AspNet.Core;
using Twilio.TwiML;

public class SmsController : Controller
{
    public TwiMLResult Index()
    {
        var messagingResponse = new MessagingResponse();
        messagingResponse.Message("The Robots are coming! Head for the hills!!");

        return this.TwiML(messagingResponse);
    }
}

Note that the this keyword is required because it's an extension method.

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.

[dprothero]

LGTM!