add check for reattach-to-user-namespace

if reattach-to-user-name-space exists, run it, otherwise just run the shell

* create login-shell command to perform this check
* update tmux.conf to run login-shell instead of reattach-to-user-namespace directly

bin/login-shell

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+shell=$(basename "$SHELL")
+
+# check if reattach-to-user-namespace is available
+if [ -n "$(command -v reattach-to-user-namespace)" ]; then
+    reattach-to-user-namespace -l "$shell"
+else
+    exec "$shell -l"
+fi