I’ve been doing this with UEStudio, but it’s Windows only. You didn’t specify your platform. No need for plug-ins, no having to set up an auto-complete file (though it supports auto-complete files). Just include the CodeIgniter App path and System path to the project, and it will “know” what you need from there, as it will parse all the files for you.
Also, it’s not free, but you get what you pay for. It’s also not horribly slow like all the eclipse based IDEs. Seems all the IDEs that are either .NET or Java based are painfully slow if you load a large project. UEStudio is quite fast, as nimble as a lightweight text editor with all the power of an IDE.
If you develop in more than one language (like the way I also create desktop applications) then it’s a real boost not to have to switch IDE for a different type of project. I can develop in PHP, HTML, JavaScript, Delphi, C++, Python, or whatever else my heart desires without switching IDEs.