Support additional ACME providers

Currently we support automatic TLS with Let’s Encrypt. It would be nice to support other providers as well. This would involve adding a deploy flag for the ACME registry URL, as well as any necessary credentials.

We have [a similar feature in Thruster](https://github.com/basecamp/thruster/pull/2) that we could probably port over.

The default should still be to use Let’s Encrypt, and we should keep the flag for using LE’s staging environment as well. The ability to use another provider would be entirely optional.

Start by reading the current automatic TLS and Let's Encrypt implementation, then compare it with the similar feature in the linked Thruster pull request. Done means an optional ACME registry URL and required credentials can be configured, while Let's Encrypt remains the default and its staging flag continues to work.

Written by the indexing model from the issue text.