Output to .json

[paulradzkov]

I've pulled and merged @mkilpatrick's and @embats's changes, so my pull request contains fix for issue #26 and has an option to save output into .json file.

Solution has been tested on this repository https:/IDTdesign/bossrev-psi

You can test it on your own project by adding direct git link to my fork:

  "devDependencies": {
    "grunt": "^1.0.1",
    "grunt-pagespeed": "git:/paulradzkov/grunt-pagespeed.git"
  }