Rewrite default urls to ./index.html

[jaredstehler]

Since we need to use a CloudFrontOriginAccessIdentity to not expose our S3 content publicly, we can't (I believe) use the S3 website endpoint as a CloudFront origin, meaning we can't use the S3 website feature which allows for default folder URLs to resolve to index.html under the hood. This PR rewrites all URI's which end in '/' to append 'index.html'.

It might be nice to make this configurable; I'm willing to go down the road of updating the build.js to add that as a parameter if folks think this is useful enough.

[ashtonian]

I think its worth making configurable :)

[thatsjet]

Original suggestion came from me and YES totally would love to have it configurable. Also I'd be totally happy to contribute the PR to modify build.js if you want.