Don't Assume Text about telnetpp

[KazDragon]

In particular, this solves the MCCP (http://tintin.sourceforge.net/mccp/) problem, whereby a compression layer sits below the Telnet layer, and its enabling layer sits above. The problem is that the IAC WILL must not be encrypted, since encryption hasn't been enabled. However, everything after the IAC WILL must be encrypted, including further Telnet conversation.

By allowing other types of data to pass through the Telnet layer, you could example register a slot with the MCCP option's activation, and return a custom tag. In this way, the tag will be passed through the activation sequence immediately following the IAC WILL, and the MCCP option can detect the tag and begin encryption during the next transmission. This would work similarly with disabling the option.

[KazDragon]

After some initial work trying to hammer in a boost::any to terminalpp::token, this makes the interface really clunky. Some investigation is required.

[KazDragon]

Came up with a telnet_pass = variant<telnetpp::token, boost::any> structure. It's still clunky, but far less so than hammering a boost::any into the telnetpp::token variant raw. All the tests pass, but it may be worth considering if there's a better interface to be had.