This PR:

• adds collection.config.addVector method to support creating new named vectors on the collection. Only named vectors can be created and existing named vectors cannot be modified.
  The method accepts the same arguments as collections.create(... { vectorizers: XXX }) with the exception that anonymous vectors are not permitted, e.g. vectorizer.none().
• fixes a bug whereby passing a vector with a custom name as a single object and passing it as an array [single object] to collections.create() would result in 2 different behaviors: custom name changed to default vs custom name preserved.

// Both cases should generate vectorConfig: { 'custom': {...} }
collections.create(..., { vectorizers: vectorizer.none('custom') });
collections.create(..., { vectorizers: [vectorizer.none('custom')] });