[zsh] add global alias `CSU` for listing unique files with `grep | grep`

usage:

```
$ grep -r TODO . | grep user
./app/views/users/show.html.slim:/     td TODO: 未実装
./app/views/users/show.html.slim:/   p TODO: 未実装
./app/models/user.rb:  include User::CommentModule # TODO: has_one を使う
./app/controllers/users_controller.rb:    # TODO: 未実装
```

```
$ grep -r TODO . | grep user CSU
./app/controllers/users_controller.rb
./app/models/user.rb
./app/views/users/show.html.slim
```