Have pretty formatter hide source uri on DEBUG_LEVEL < 3

A recent change in cucumber.js has caused the pretty formatter to always output a source uri before every feature and step:

Feature: some feature
        Scenario: I haven't done anything yet   # features\a.feature:2
          Given I have not defined this step    # features\a.feature:3

This caused our CI server to output a lot more text than necessary (the path names are very long).

This pull request allows you to change whether or not these source uris are printed out using the DEBUG_LEVEL environment variable.

The default behaviour is still to output these lines, but if you reduce the debug level to 1 or 2, they will stop printing.

The pull request also includes cucumber tests for this new functionality under features/pretty_formatter.feature