Add `identifier` prop for seeking timeline to a certain animation

[wintercounter]

Is your feature request related to a problem? Please describe. I'm having a pretty huge timeline. Now I'm saving the percentages to an array of each scene. Every time I modify a duritaion/delay, I always need to update this array.

Describe the solution you'd like I'd like to be able to add an identifier property and use that for seeking.

MyTimeline.add({
     identifier: 'section3'
})

MyTimeline.seek('section3')