[steeef]

You're thinking about xcape: https://github.com/alols/xcape.

FYI, Karabiner is incompatible with MacOS Sierra. There's a project (Karabiner-Elements) to replace its (and Seil's) functionality, but I don't believe it has feature parity yet: https://github.com/tekezo/Karabiner-Elements

[karmajunkie]

Wow thanks for the heads up... that would have been a shitty thing to discover the hard way!