feat(mocha): add support for extra reporters

[delatrie]

Context

The PR makes it possible to enable additional reporters to run alongside Allure Mocha.

Examples

The feature is controlled by a new configuration property:extraReporters. Its values may take the following forms:

• A string:

  { extraReporters: "json" }

  This is usable from the CLI or config files:

  npx mocha -R allure-mocha -O extraReporters=json

  .mocharc.json:

  {
    "reporter": "allure-mocha",
    "reporterOptions": [
      "extraReporters=json"
    ],
  }

• An array of one or two elements, where the second element (if present) is an options object:

  { extraReporters: ["json", { output: "output.json" }] }

• An array of elements, each taking one of the above forms:

  {
    extraReporters: [
      ["json", {output: "output.json"}],
      "mochawesome",
    ],
  }

Closes #1134.

Checklist

• Sign Allure CLA
• Provide unit tests