Custom labels

Fix

• Fix too much logs (Issue 5)

Enhancements

• You can now provide your custom time widget (Request 3):

TimelineEditor(
  timeWidgetExtent: 100,
  timeWidgetBuilder: (d, t) => Padding(
            padding: const EdgeInsets.only(left: 8.0),
            child: Text(
              '${d.inSeconds}/${t.inSeconds}',
              overflow: TextOverflow.ellipsis,
            ),
          ),