Let's Encrypt support

[technosophos]

While this isn't as pressing as #188, we do at some point want to have Let's Encrypt support.

Ideally, what we would want, UX-wise, is the user to choose their domain, and then indicate that they want SSL certificates generated for them. This would likely require the user to do something with a TXT record in their DNS provider.

[bacongobbler]

let's see if this is feasible for 0.2.

[bacongobbler]

For YarpReverseProxy: https://github.com/natemcmaster/LettuceEncrypt

[bacongobbler]

Hmm... Doesn't look like we're able to issue separate certificates for each application at this time: natemcmaster/LettuceEncrypt#236

The project is in maintenance mode, so it's unlikely this feature will be implemented unless we spend the time to contribute this feature. Also judging by natemcmaster/LettuceEncrypt#236 (comment) it's unlikely it'll be merged if we contribute a PR...

[bacongobbler]

deferring to 0.3.0 in favour of #384.

[benschop-it]

I have implemented the separate certificates feature in my fork at https://github.com/benschop-it/LettuceEncrypt-Benschop-IT.
This makes it possible to create a certificate per set of domains.
Some extra tests still need to be added.
Feel free to comment.