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_defs.c

@@ -695,14 +695,16 @@ static struct cmd_t command_defs[] = {
       CMD_SYN(
               "/affiliation set <affiliation> <jid> [<reason>]",
               "/affiliation list [<affiliation>]",
-              "/affiliation request")
+              "/affiliation request",
+              "/affiliation register")
       CMD_DESC(
               "Manage room affiliations. "
               "Affiliation may be one of owner, admin, member, outcast or none.")
       CMD_ARGS(
               { "set <affiliation> <jid> [<reason>]", "Set the affiliation of user with jid, with an optional reason." },
               { "list [<affiliation>]", "List all users with the specified affiliation, or all if none specified." },
-              { "request", "Request voice."})
+              { "request", "Request voice."},
+              { "register", "Register your nickname with the MUC."})
       CMD_NOEXAMPLES
     },