Add option report-json

[tyankatsu0105]

Do you want to request a feature or report a bug?
feature

What is the current behavior?
Report function does not exist.

If the current behavior is a bug, please provide the steps to reproduce.

What is the expected behavior?
When run npm run bundlesize --output-json,
output bundlesize-report.json.

[
  {
    "message": "./dist/js/about.93459521.js: 282B < maxSize 10KB (gzip)",
    "fail": false,
    "size": 282,
    "maxSize": 10240
  },
  {
    "message": "./dist/js/app.0b7c728b.js: 2.21KB < maxSize 10KB (gzip)",
    "fail": false,
    "size": 2264,
    "maxSize": 10240
  },
  {
    "message": "./dist/js/chunk-vendors.f167eeb4.js: 40.63KB > maxSize 10KB (gzip)",
    "fail": true,
    "size": 41608,
    "maxSize": 10240
  },
  {
    "message": "./dist/css/app.c6167d4a.css: 271B (gzip)",
    "fail": false,
    "size": 271,
    "maxSize": null
  },
  {
    "message": "./dist/img/logo.82b9c7a5.png: 11.44KB (no compression)",
    "fail": false,
    "size": 11713,
    "maxSize": null
  }
]

This option is always exit code 0.
So --report-json option is not always an error.

If this is a feature request, what is motivation or use case for changing the behavior?
We can edit data made from bundlesize

Please mention other relevant information.

• node version 11.8.0
• npm version 6.5.0
• Operating system nodenv
• bundlesize version 0.18.0
• CI you are using TravisCI