implement certificate aliases as suggested by typingArtist (fixes #396)

docs/domains_txt.md

@@ -7,7 +7,13 @@ The file should have the following format:
 ```text
 example.com www.example.com
 example.net www.example.net wiki.example.net
+example.net www.example.net wiki.example.net > certalias
 ```
 
 This states that there should be two certificates `example.com` and `example.net`,
 with the other domains in the corresponding line being their alternative names.
+
+You can define an alias for your certificate which will (instead of the primary domain) be
+used as directory name under your certdir and for a per-certificate lookup.
+This allows multiple certificates with identical sets of domains but different configuration
+to exist.

docs/per-certificate-config.md

@@ -16,3 +16,10 @@ Currently supported options:
 - WELLKNOWN
 - OPENSSL_CNF
 - RENEW_DAYS
+
+## DOMAINS_D
+
+If `DOMAINS_D` is set, dehydrated will use it for your per-certificate configurations.
+Instead of `certs/example.org/config` it will look for a configuration under `DOMAINS_D/example.org`.
+
+If an alias is set, it will be used instead of the primary domain name.