tmux: update config for tmux 2.1
tmux: update config for tmux 2.1

I just updated to the current tmux `master` branch (which is pre-2.1, I
believe), in order to try to better mitigate this Apple issue on El
Capitan:

https://github.com/tmux/tmux/issues/108#issuecomment-148160119

tmux has dropped support for `c0-change-trigger` and
`c0-change-interval` in 44364d7112037bebb0bb1d2:

> Remove the c0-* options which never really worked satisfactorily. Going
> to try something else...

Meanwhile, several mouse options have gone away and equivalent
functionality is exposed using just `set -g mouse on`.

Known glitch: italics are borked. Going to investigate, but at a quick
glance looks like it's explained by 7382ba82c5b366be84ca55c78:

> If default-terminal is set to "screen" or "screen-*", emulate screen's
> historical (incorrect) behaviour for SGR 3 and send smso
> (standout). Previously, we would send sitm (italics) if the terminal
> outside had it and smso otherwise. This was acceptably until recently
> because xterm's terminfo entry lacked sitm, so most users got smso.
>
> People who want italics should set default-terminal to the forthcoming
> "tmux" entry (and be prepared to deal with it being missing on older
> hosts).
>
> As a side-effect this changes default-terminal to be a server rather
> than a session option.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent