Accept user settings as environment variables

[shazow]

Something like:

$ SSHCHAT_TIMESTAMP=1 SSHCHAT_THEME=mono ssh -o SendEnv "SSHCHAT_TIMESTAMP SSHCHAT_THEME" chat.shazow.net

or with an ssh_config:

Host chat.shazow.com
    SendEnv SSHCHAT_TIMESTAMP SSHCHAT_THEME

And with SSHCHAT_TIMESTAMP and SSHCHAT_THEME exported properly, then normal connection would work.

This would be equivalent to connecting and running:

/timestamp
/theme mono

This is related to #104 because something like TERM=bot should be equivalent to /theme mono.

[shazow]

This was done in #333.

Need to document this somewhere, maybe in the wiki or README? (How to setup a nice ssh_config)

[shazow]

Done by @sleibrock at https://github.com/shazow/ssh-chat/wiki/Environment-Variables, thanks!