Add additional features to IRC input

Original PR: https://github.com/elasticsearch/logstash/pull/1800

This PR adds the ability to collect more details (JOINs and PARTs) and add's the optional ability to request a NAMES list on X interval, thus giving statistics on number of users in channels. 


We have some redundancy here, we are checking the boolean value of `@get_stats` in  two differents conditionals.

We can replace it with one:

```ruby
if @get_stats
  if msg.command.to_s == RPL_NAMREPLY
  #...
  elsif msg.command.to_s == RPL_ENDOFNAMES
  #...
  end
end
```