Adding support for private confs

First off, I typically don't write C code so I apologize for anything that's obviously wrong. :)

I love the client, but was unable to join private rooms. I worked through adding the ability to join chats that require a password, and have been able to successfully use it, but there are a couple things that I'm unsure about:

1. If you need to specify a password, you *need* to provide a nick to `/join` to connect.
2. I saw that the bookmarks feature calls the same functions, so I added a `NULL` parameter. I've never worked with variadic functions (right? those with variable numbers of parameters?) in C, so I felt this was less likely to break functionality. 
3. Additionally, I added no test for this functionality. I'm happy to, if I can get a little bit of direction on it...