~/.tmux.conf: Support mouse scrolling.

It's kind of inconvenient because it doesn't allow normal mouse
selection, but we'll see how it works out.

.tmux.conf

# [stolen tmux.conf]: https://gist.github.com/1689941#file_.tmux.conf
# [patched fonts]: https://github.com/Lokaltog/vim-powerline/wiki/Patched-fonts
set -g mode-mouse on
set -g default-terminal "screen-256color"
set -g status-utf8 on
set-option -g set-titles on