Can support clone mode

[xiaoException]

Hi mfuu,
Thank you for fixing the last issue v-model.

Recently I used the latest version and ran into another problem. In contrast to vue-draggle component library, which can support pull=clone mode but can't support virtual scrolling, vue-virtual-drag-list is a component that allows list groups to be cloned without adding new elements (put:false,pull:'clone'). Another list group can drag elements can also add elements (put: true/add, pull: true/remove).

See if you need to enhance the functionality of this component

[mfuu]

sorry for late reply, please update to the latest version and use `pull: 'clone'´ in group.