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

```
Merge branch 'master' of https://github.com/joshprice/stubby4node into joshprice-master
reducing computational load for applyCaptures in file
Merge branch 'joshprice-master'