feat(timeline): add custom events for markers

[ryamaguchi0220]

Closes #131

This PR adds two custom events

• markerchange: fired when the marker title has been changed.
• markerchanged: fired when an alteration to the marker title is committed.

Demo
https://visjs.github.io/vis-timeline/examples/timeline/markers/markerEvents.html

[yotamberk]

Nice job. I do feel though that the event names are unclear. I think they should be called onMarkerInputChange and onMarkerInputChanged.

[ryamaguchi0220]

I do feel though that the event names are unclear

I named the custom events based on change event and input event because I think they are familiar to web developers. If change the names, I propose onMarkerChange and onMarkerChanged or onMarkerTitleChange and onMarkerTitleChanged. What do you think about it?

[yotamberk]

Sounds good enough to me the new names you offered

[ryamaguchi0220]

I've renamed the custom events.