Description of the desired feature

As discussed in #175, it would be nice to have a Pooch.clear_cache method to delete all contents of the local storage. This would have to call shutil.rmtree on `Pooch.abspath

The main s3cr3t site should:

• Accept TLS certificates and keys
• Redirect all HTTP traffic to HTTPS

Sounds like we need to modify the Dockerfile and add some new arguments (key, cert, and tls_enable), and if tls_enable is set, the site should change accordingly.