Add support to register with a room

`/affiliation register` can now be used to register a nickname with a
MUC.

Tested with a server without forms. Couldn't find a server which
supports forms yet.

Implements https://github.com/profanity-im/profanity/issues/1210

src/command/cmd_ac.c

@@ -756,6 +756,7 @@ cmd_ac_init(void)
     affiliation_cmd_ac = autocomplete_new();
     autocomplete_add(affiliation_cmd_ac, "list");
     autocomplete_add(affiliation_cmd_ac, "request");
+    autocomplete_add(affiliation_cmd_ac, "register");
     autocomplete_add(affiliation_cmd_ac, "set");
 
     role_cmd_ac = autocomplete_new();