~/.tmux.conf: Set the application name as the terminal title

This makes for prettier log messages in ~/.eventstream (not just urxvt
there).

.tmux.conf

set -g default-terminal "screen-256color"
set -g status-utf8 on
set-option -g set-titles on
set-option -g set-titles-string "tmux(#I): #W"
set -g status-bg black
set -g status-fg white