irssi: add module

[CRTified]

I wanted to switch to a CLI-based IRC client. As I did not see anyone available as a module, I decided to write one.
I plan to make more options configurable, so it might make sense to wait with the merge.
So far, this module allows basic configuration of the chatnets, servers and channels. As an example config:

    programs.irssi = {
       enable = true;
       aliases = {J = "join"; BYE = "quit"; };
       chatnets = {
         freenode = {
           type = "IRC";
           nick = "FOO";
           autoCmd = [ ];
           channels = [
             { name = "#nixos"; autojoin = true;}
             { name = "#nixos-chat"; autojoin = false;}
             ];
           server = {
             address = "irc.freenode.net";
             port = 6697;
             useSSL = true;
             verifySSL = true;
             autoconnect = true;
           };
         };
       };
    };

As far as I understood, irssi allows the usage of plugins to add other network types (i.e. XMPP). So far, chatnets.<name?>.type is not checked for validity.

Note that this is my first pull request, so I'd love some feedback :) I hope that I understood the syntax guidelines correctly.

[teto]

cool though I would have preferred to see a weechat one xD

[rycee]

Thanks for the contribution! Looks good in general, I've just added a few comments.

[rycee]

Thanks! I've squashed and rebased into master in 02a5a67. I made a few slight alterations of minor things I noticed when doing the merge, like removing the unnecessary serverType and adding an example network.