First split into crockery init and crockery run

The init command creates a crockery.db file containing the domain name and
TLS keypair. The run command starts IMAP and SMTP services based on that file.

Supporting only a single domain is starting to look a bit unnecessary. We'll
see how that goes.

DESIGN.md

@@ -1,5 +1,7 @@
 # Crockery design
 
+## Overview
+
 We need:
 
 * SMTP server
@@ -18,4 +20,23 @@ We need:
   * Search emails - some sort of inverted index necessary
     * https://github.com/blevesearch/bleve ?
   * Embedded database best. Ideally we have a single file to work with
+    * https://github.com/coreos/bbolt
+    * https://github.com/asdine/storm
 
+## Database structure
+
+```
+config/
+  domain: "example.com"
+domains/
+  example.com/
+    accounts/
+      foo@example.com/
+        config/
+        emails/
+    config/
+      key: []byte(...)
+      cert: []byte(...)
+    
+
+```