Add command for numeric reply

cf. http://www.haun.org/kent/lib/rfc1459-irc-ja.html#c2.4

## Sample
```ruby
require 'zircon'

client = Zircon.new(
  server: '127.0.01',
  port: 6667,
  channel: '#channel',
  username: 'dummy',
)

client.on_numericreply do |msg|
  puts "(#{msg.type}) #{msg.body}"
end

client.run!
```

## Output
```
(001) Welcome to the debian Internet Relay Chat Network dummy
(002) Your host is hybrid7.debian.local[127.0.0.1/6667], running version hybrid-7.2.2.dfsg.2-6
(003) This server was created Feb  8 2010 at 21:14:28
(004) hybrid7.debian.local
(005) CALLERID
(005) CHANLIMIT=#&:15
(251) There are 0 users and 3 invisible on 1 servers

...
```