HSTS Support

In addition of the HTTPS redirect, adding HSTS support will even skip this redirect the next time a supporting browser asks for a non-secured resource.

We potentially gain an HTTP query per non-HTTPS requested resource and smoothly increased security and privacy.

The value is set accordingly to the recommended value in the spec.

If you agree with this proposal, the same change can be propagated in other boilerplate files (like Nginx etc.).

It's a follow-up of h5bp/html5-boilerplate#1365