Add functionality to swap in custom disambiguators

[beckydvn]

Allow the bot designer to add custom disambiguators to differentiate between >1 Spacy entity in the same utterance (see #1). In the future, this would apply not only to Spacy but any extraction method that would cause ambiguity (Rasa included).

For example, in the utterance "I want to fly from London to Berlin," Spacy would extract two GPE entities: London and Berlin. However, without extra help it is unable to tell you which is the origin city and which is the destination. A very basic custom disambiguator would assign London to the origin city as it is preceded by "from" and Berlin to the destination as it is preceded by "to."

While it is true that Rasa has "roles" and "groups" functionality to address this, binding dialogue disambiguation to Rasa that tightly significantly complicates things, given that not all entities are extracted with Rasa. Additionally, Hovor has no notion of "roles" or "groups" and changes would have to be made to accommodate that. Separate context variables would be needed in the end, which defeats the purpose of Rasa's roles/groups of removing the need for "duplicate variables." So, some functionality detached from the extraction method is needed to handle cases like these.