Introduce transformPasted event
Introduce transformPasted event

To transform pasted content after it has been parsed into ProseMirror's
representation.

Closes #221

tzuhsiulin/prosemirror,mattberkowitz/prosemirror,mattberkowitz/prosemirror,adrianheine/prosemirror,tzuhsiulin/prosemirror,kiejo/prosemirror,salzhrani/prosemirror,adrianheine/prosemirror,adrianheine/prosemirror,kiejo/prosemirror,salzhrani/prosemirror,salzhrani/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror