Add support for identify-msg

Some IRC networks support the `identify-msg` capability. This prepends all `PRIVMSG` messages with a `+` or  `-` character to indicate whether the user is identified with network services or not.

This patch requests the capability from the network, and if present, parses these characters, and passes the status through to commands via the `SAXO_IDENTIFIED` environment variable.

I have verified that this works with commands. But have not verified that it works for plugins. Though, I added an additional class property on the `ThreadSafeEnvironment` class as requested.