Support search history

I added support for search history which moves autoload/history.vim into autoload/history/cmd.vim and adds autoload/history/search.vim. It also renames CtrlPHistory to CtrlPCmdHistory (to make the command unambiguous).

Not sure if you want these changes, but I think the search history is pretty neat. Not sure if there's a better way to make these two very similar things, so I just copied all of it (looks like almost everything is boilerplate config code).