node-yjs binding to a CodeMirror editor

 node-y-codemirror binds a Y.Text to a CodeMirror editor.
 Features:
 * Sync CodeMirror editor
 * Shared Cursors
 * Shared Undo / Redo (each client has its own undo-/redo-history)
 * Successfully recovers when concurrents edit result in an invalid
   document schema