Add movement speed elixir so people can buy 6 items

In games that go past 40 minutes, I am usually maxed on items, sometimes with over 3k+ gold sitting around. Selling boots on many champions is not viable because they need the movement speed too much. If there was a movement speed elixir, I could sell my boots and get a 6th item. The elixir could also be interesting on other champions that like movement speed a lot.