Introduce nb field for capabilities

[cdata]

Part of #11

https://github.com/ucan-wg/spec#241-nb-non-normative-fields

[jsantell]

nb has been removed in 0.10.0. The capabilities are currently defined as:

{
    "att": [{ "with": "$RESOURCE", "can": "$ABILITY", "nb": "$UNUSED" }]
}

As of 0.10.0, att should be renamed to cap, with a structure like:

{
  "cap": {
    "$RESOURCE":  {
      "$ABILITY_1": [{}],
      "$ABILITY_2": [{ "foo": "bar" }]
    }
  }
}