Option to hide drag handle based on a condition

[cchandurkar]

I want to hide the drag handle based on conditions like empty paragraphs, a special checklist, etc.

Two potential ways of doing it could be:

1. Add shouldShow: (node: Node) => boolean config method that checks if the handle should be shown.
2. Replace excludedTags[] with excludedSelectors[] that can take a qualifying DOM selector such as p.is-empty or ul.special-checklist etc.

[cchandurkar]

Edit: I think the p.is-empty already works in excludeTags[].