Add Enabling Vi Mode as a tmux til.

tmux/enabling-vi-mode.md

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+# Enabling Vi Mode
+
+If you'd like some vim-like bindings in tmux, you can turn on vi mode. To do
+so, add the following line to your `.tmux.conf` file:
+
+```
+setw -g mode-keys vi
+```
+
+With this added and tmux re-sourced, you'll now have a variety of vi-like
+bindings. For instance, when in tmux's copy mode, you can now use `v` to
+begin making a visual selection and `y` to yank that selection into tmux's
+copy buffer.