Add allow_field option

## What does this Pull Request Do?

Add allow_field option

## How should this be manually tested?

Run the test

## Any background context you want to provide?

It's a list of fields allowed for work, for default all fields are allowed.

## What are the relevant tickets?

Express Collector for Oasis not showing all hashtags

## Any extra info?

@xeBuz 

```results[0].lastname``` should be 'Galt'
if you do ```allow_fields = ['bananas']``` nothing should happen, and return None