Add reporters as plan options

[jamlen]

Support adding reporters at plan and job level. This will also affect the --summary and --summary-format command line options, which should set the appropriate report up as a default.

Something like:

  reporters:
    -
      type: Summary
      output: ./summary.json
      format: json
    -
      type: Detailed
      output: ./report.html
      format: html

This allows other reports to be easily added later whilst still giving the user the control over which ones they want and where.