Bucket construct should expose WebsiteURL

[AlexChesters]

Currently, (as far as I can see), the Bucket construct doesn't export the WebsiteURL property. It would be useful if this was exported to prevent situations like the following where you have to hardcode bits of a URL

import * as cloudfront from '@aws-cdk/aws-cloudfront'
import * as s3 from '@aws-cdk/aws-s3'
import * as cdk from '@aws-cdk/cdk'

export default class Stack extends cdk.Stack {
  constructor (parent: cdk.App, name: string, props?: cdk.StackProps) {
    super(parent, name, props)

    const bucket = new s3.Bucket(this, 'Bucket', {
      publicReadAccess: true,
      websiteIndexDocument: 'index.html'
    })

    const distribution = new cloudfront.CloudFrontWebDistribution(this, 'Distribution', {
      aliasConfiguration: {
        acmCertRef: '<MY_CERT_ARN>',
        names: ['my.domain']
      },
      originConfigs: [
        {
          behaviors: [
            {
              defaultTtlSeconds: 10,
              isDefaultBehavior: true,
              maxTtlSeconds: 30,
              minTtlSeconds: 0
            }
          ],
          customOriginSource: {
            // ideally here we could do something like:
            // domainName: bucket.websiteURL,
            domainName: `${bucket.bucketName}.s3-website-eu-west-1.amazonaws.com`,
            originProtocolPolicy: cloudfront.OriginProtocolPolicy.HttpOnly
          }
        }
      ]
    })
}

Planning to open a PR for this unless someone is already working on it :)

[AlexChesters]

Opened #1522 #1544 for this