Several improvements

[krisbreuker]

For our CMS we came across this ACME client, which seemed promising. For our use there were some features missing though, that I implemented in a fork of this repo. Specifically, I've added support for

• External Account Binding using a kid and hmac key
• HTTP challenges
• Some wildcard support (enough to get it working)
• RSA key pairs for certificate signatures
• Skipping challenges altogether (not all providers require these)

I'm curious if you might be interested in these changes and if so, if I should open a single pull request for everything or one pull request for each feature (although some of them are dependent on others).

Thank you for your work!

[ycmjason]

Hey! First of all, it means the world to me to know that someone is actually using this (or attempting to use this)!

Thanks so much for helping improve it. If possible, could you submit a separate PR for each feature? It just makes it easier to review things with a bit more focus.

Thanks again!!!