How to use:

(Markers Mode)

Select the marker you want to link and press on bind. This will bind any keyframes that are on the same spot as the marker, and now when you move the marker, the keyframes will follow.

If you try to move the bound keyframes, they will get locked. However, you can move other keyframes that are not bound freely.

If you place a linked marker on top of other keyframes, the bound keyframes will wait until the marker goes into a free spot and follow back again.

If you delete the marker, or press unbind, the link will be erased.

(Keyframes Mode)

This mode is usually used for scaling and duplicating the linked keyframes.

If you move any keyframes that are linked, their markers will follow.

if you delete the keyframes, or place them on top of other linked keyframes, the link will be erased.

Display All Points: Shows hidden and unhidden points of all objects in the scene.

In order for the markers to follow keyframes while moving the keyframes, all keyframes must be selected. 

Select Column: By selecting one keyframe point, this tool selects all the other points in one column automatically. In keyframes mode markers will not sync unless all the keyframes moved, and this can eliminate some of the hassle of not selecting all the points accurately.

New feature added:

Hovering over properties panel also unlocks the animation to enable editing properties while maintaining locked keyframes to markers.

Disclaimer: Added Grease Pencil support ! documentation is coming soon...

Known Bug: scaling the keyframes in the dopesheet works perfectly unless a number input is used. For example: if two keypoints at frame 1 and 10 where scaled with s and a number was inputted, the markers would lose link. This bug is not critical due to the fact that the user can still scale the keypoints with the mouse and everything would work fine. But its worth to note that due to the significance of performance loss fixing this bug would cause, fixing it is postponed until further notice.