Atom already has this integrated, not as a plugin, but right into the editor. I cannot recall where at the moment, but you can get a list of every package, and it'll list the startup time in milliseconds.
Timecop - but it has a huge weakness in that the startup time before the main window appears is not counted. I can use a stopwatch and get about 8 seconds, while timecop only shows me ~1100ms, and plugins account for maybe ~100ms of that.