Feature request: ACL-list

Firstly; many thanks for this project, looks really promising.

I haven't started using this yet, but plan on implementing it. I've read through the documentation but can't find anything relevant to this.

Basically what I would like, is the ability to close access to the proxy except where I've explicitly granted access for certain CIDR-notation, and optionally scope exactly which DNS:es this CIDR-notation is allowed to request a certificate for.

I'm thinking something like a `acl.json` perhaps, just as proposal:

``` json
{
"10.1.2.3/28": [""],
"10.2.3.4/32": [""],
"10.3.4.5/29": [
"this.is.allowed.com",
"this.also.com",
],
}
```

Start by reviewing the existing documentation and the proxy's current access and certificate-request behavior. Define how CIDR-based access and optional DNS restrictions should be represented in acl.json, including the meaning of an empty list, then document acceptance cases for allowed and denied requests.

Written by the indexing model from the issue text.