Presence

[curran]

There's been a new presence implementation added in ShareDB.

Here's an example of an OT type extended to support presence, compatible with ShareDB: ottypes/rich-text@ce14c8f#diff-4e2abe5a12642efc78b26087a61a8e9f

The task at hand with this issue is to come up with a transformPresence function for text-unicode, and also design the presence object shape.

Strawman proposal for presence object shape (inspired by that of rich-text):

{
  selections: [ // Let's say we support multiple selections in text-unicode
    {
      index: 4, // The index in the string where the start of the selection is.
      length: 3 // The length of the selection (number of characters long).
    },
    { start: 9, length: 2 }
  ]
}