Simple as it is, this mod adds the functionality to right click any full grown crop to harvest it. It will automatically replant a new crop. This mod needs to be installed on server only, obviously on client if you play singleplayer.
There are quite a few mods out there which implement this feature, additionally to all their other features. This mod is just a standalone if you don't want to install a mod with much more content, or want a server where vanilla clients may connect to.