[WIP] Use `vi`-like keybindings, when copy-mode.
Use `vi`-like keybindings, when using copy-mode.

```
diff --git a/.tmux.conf b/.tmux.conf
index 229243b..29eff2c 100644
--- a/.tmux.conf
+++ b/.tmux.conf
@@ -42,6 +42,9 @@ bind l select-pane -R
 unbind Tab
 bind Tab select-pane -t :.+

+# vi-like keybindings, when copy-mode
+set-window-option -g mode-keys vi
+
 # ------------------------ #
 # mouse mode
 # ------------------------ #
```
Merge pull request #26 from kazu634/tmux-copy-mode

Use `vi`-like keybindings, when tmux copy-mode.