Dynamic token replacement in file name
Dynamic token replacement in file name

Allows dynamic file lookup based on regex captures

ie 

```yaml
- request:
    url: ^/api/endpoint$
    query:
      scenario: ([-a-z]+)?
  response:
    file: <% query.scenario[1] || "default" %>.json

```
kombucha/stubby4node,apchamberlain/stubby4node,codeorganic/stubby4node,apchamberlain/stubby4node,Droogans/stubby4node,kombucha/stubby4node,Droogans/stubby4node,codeorganic/stubby4node