Search tool for Safari
I know what you want because I had that in Opera a long time ago, but then I switched to Mac full time and dropped Opera. Safari lacks in that aspect. 

I don’t know if there’s a specific extension for that, but you might want to take a look at tools like LaunchBar ($$) or Quicksilver (Free). 

I use Launchbar and I type ‘goo’ and then whatever I want to type and enter. I Have searches for StackOverflow, Wikipedia, Amazon, etc. You can add whatever you want. 

After some time doing that, it becomes extremely much more natural, because you can invoke either of those from anywhere in the system. You just invoke Launchbar, goo + xxxxx and you’re done. 

Give it a try, I believe is a much more natural workflow than having to find safari, going to the search box and typing there.