Add completion to new-session and new-window commands

Complete names of existing sessions and windows for the `new` commands,
enabling easier creation of similarly named layouts.

libexec/tmuxifier-new-session

@@ -2,6 +2,14 @@
 set -e
 [ -n "$TMUXIFIER_DEBUG" ] && set -x
 
+# Provide tmuxifier completions
+if [ "$1" == "--complete" ]; then
+  for item in $(tmuxifier-list-sessions); do
+    echo "$item"
+  done
+  exit
+fi
+
 if [ -z "$1" ]; then
   echo "$(tmuxifier-help new-session)" >&2
   exit 1