Disabling mutating "true" -> "false" in TrueValue mutator for in_array/array_search

[maks-rafalko]

With this PR, TrueValue mutator will no longer mutate true to false for in_array and array_search functions. This can be explicitly enabled in settings.

{
  "timeout": 25,
  "source": {
    "directories": ["src"]
  },
  "mutators": {
    "TrueValue": {
      "settings": {
        "in_array": true, // default "false"
        "array_search": true // default "false"
      }
    }
  }
}

Also, add json schema validation for TrueValue mutator in infection.json.

• Covered by tests
• Doc PR: Add documentation for TrueValue mutator setting site#105

Fixes #578
Fixes #206

[sanmai]

Overall looks sensible to me.

[maks-rafalko]

@BackEndTea could you please review again?

[maks-rafalko]

thank you @BackEndTea @sanmai @localheinz