Adding contributors wreck

[Berkmann18]

• all-contributors-cli version: latest
• node version: 11.12.0
• yarn version: 1.16.0

Relevant code or config

{
  "projectName": "cz-cli",
  "projectOwner": "commitizen",
  "repoType": "github",
  "repoHost": "https://github.com",
  "files": [
    "AC.md"
  ],
  "imageSize": 100,
  "commit": false,
  "commitConvention": "angular",
  "contributors": [
    {
      "login": "jimthedev",
      "name": "Jim Cummins",
      "avatar_url": "https://avatars0.githubusercontent.com/u/108938?v=4",
      "profile": "https://jimthedev.com",
      "contributions": [
        "review"
      ]
    },
    {
      "login": "marionebl",
      "name": "Mario Nebl",
      "avatar_url": "https://avatars1.githubusercontent.com/u/4248851?v=4",
      "profile": "https://github.com/marionebl",
      "contributions": [
        "review"
      ]
    },
    {
      "login": "vagisha-nidhi",
      "name": "Vagisha Nidhi",
      "avatar_url": "https://avatars1.githubusercontent.com/u/13175100?v=4",
      "profile": "https://www.linkedin.com/in/vagisha-nidhi-891766126/",
      "contributions": [
        "review"
      ]
    },
    {
      "login": "plroebuck",
      "name": "P. Roebuck",
      "avatar_url": "https://avatars1.githubusercontent.com/u/16603429?v=4",
      "profile": "https://github.com/plroebuck",
      "contributions": [
        "review"
      ]
    },
    {
      "login": "giocodes",
      "name": "Sergio Gomez",
      "avatar_url": "https://avatars1.githubusercontent.com/u/19365690?v=4",
      "profile": "https://giocodes.github.io/portfolio/",
      "contributions": [
        "review"
      ]
    },
    {
      "login": "LinusU",
      "name": "Linus Unnebäck",
      "avatar_url": "https://avatars0.githubusercontent.com/u/189580?v=4",
      "profile": "http://linus.unnebäck.se/",
      "contributions": [
        "review"
      ]
    },
    {
      "login": "thierrymichel",
      "name": "Thierry Michel",
      "avatar_url": "https://avatars2.githubusercontent.com/u/806883?v=4",
      "profile": "http://thierrymichel.net",
      "contributions": [
        "review"
      ]
    }
  ],
  "contributorsPerLine": 7
}

What you did:
Ran ../all-contributors-cli/src/cli.js fetch

What happened:
Things seem to work properly until the JSON data read from the configPath becomes 0 after a few addContributors calls.

Reproduction repository:
Branch ml on this one

Problem description:
Either an I/O fail on the part of fs or a bad approach regarding adding multiple contributors (each one by one).

Suggested solution:
Having a function that can handle adding several contributors at once.

[Berkmann18]

This doesn't seem to be an issue anymore on #196 so I'll close this.