Add `gim` shell function: `git ls-files . | peco`

```
diff --git a/.zshrc b/.zshrc
index 739e385..3621829 100644
--- a/.zshrc
+++ b/.zshrc
@@ -420,3 +420,9 @@ if which peco > /dev/null; then

   bindkey '^@' peco-cdr
 fi
+
+# === gim: `git ls-files` + `peco` ===
+# see: http://qiita.com/la_luna_azul/items/7998abd0379e8a3248f4
+function gim() {
+  vim `git ls-files . | peco`
+}
```