Add cut feature

[hasanbalci]

Currently, this extension allows copy and paste operations in its API. We can add a cut function as well, so that users will be able to cut and paste a graph component. Some cases to consider during this operation:

• If selected elements contain compound nodes, their descendants should also be included to cut operation.
• If there are some edges that connect elements to be cut to other elements, such edges should be removed.
• beforeCut and afterCut options should be added (see current options).
• This operation should be undoable.
• Update README and demo file to show this new feature.

[hasanbalci]

Cut feature is now added to the extension. Thanks @AsmaJodeiri.