add basic drop support for images

packages/tiptap/src/index.js

@@ -2,3 +2,4 @@ export { default as Editor } from './components/editor'
 export { default as Extension } from './utils/extension'
 export { default as Node } from './utils/node'
 export { default as Mark } from './utils/mark'
+export { default as Plugin } from './utils/plugin'

packages/tiptap/src/utils/plugin.js

@@ -0,0 +1,3 @@
+import { Plugin } from 'prosemirror-state'
+
+export default Plugin