tmux: Explicitly set vi bindings

The default is emacs, unless EDITOR contains "vi", but there's no
harm in setting this explicitly.

.tmux.conf

@@ -76,6 +76,9 @@ bind '"' choose-window
 # use Escape for entering copy mode, like screen
 bind Escape copy-mode
 
+# use vi-style key bindings
+set-option -g mode-keys vi
+
 # Rather than constraining window size to the maximum size of any client
 # connected to the *session*, constrain window size to the maximum size
 # of any client connected to *that window*. Much more reasonable.