[enhancement] Improve the format of the property "entitiesIndex"

[thiagodp]

Given a entity foo and a sentence recognized as "A {foo} and a {foo} with a {bar}.", the property entitiesIndex (of the object returned by the test() method) will be something like:

{
  "foo": { position: 2, entity: "foo", ... },
  "foo1": { position: 15, entity: "foo", ... },
  "bar": { position: 24: entity: "bar", ... }
}

The second foo entity is represented by foo1. I think it is better having an array for each entity found:

{
  "foo": [ { position: 2, entity: "foo", ... },{ position: 15, entity: "foo", ... } ]
  "bar": [ { position: 24: entity: "bar", ... } ]
}

This makes it easier to get all the results of a certain entity as well as to count them.