Use wildmenu when tab completing filenames in vim

Using `wildmenu` prevents the screen from "sliding up" when tab completing filenames (via `:e ` or `:b `). It looks cleaner, but has a small disadvantage -- it can display a limited amount of files without further input.

![screen shot 2014-05-07 at 7 17 49 pm](https://cloud.githubusercontent.com/assets/116327/2911223/ffaa6448-d656-11e3-8c46-21b13a2d3544.png)