The headphone port of the MacBook Retina 13", Mid 2014 has support for the Apple iPhone headset, see: https://support.apple.com/kb/sp703?locale=en_US
Any plug with the three black rings will do to separate the microphone and line-out.
Headphone port
- Support for Apple iPhone headset with remote and microphone
- Support for audio line out (digital/analog)
And that means, it can be used to connect an input splitter like this one:
2x 3-Pin 3.5mm Headset Splitter Adapter - M/F
http://www.amazon.com/dp/B004SP0WAQ).
You could also use an audio interface like the iMic.
It will turn a USB port into a line-in port:
Recording with iMic in GarageBand