Add a type-in completion box on home page

[bronson]

I suppose the first thing most people want to do when they arrive at vim-scripts is look up a script. Make this as easy as possible!

[bronson]

This is less important now that gmarik made the script list filterable: http://vim-scripts.org/vim/scripts.html

Still, it would be nice to have instant satisfaction on the home page if possible... Close this in 6 months if we don't figure out how. :)