Add beautifiers property to OptionValues interface

[Glavin001]

Describe the bug

To Reproduce
Steps to reproduce the behavior:

const options: OptionValues = {
  beautifiers: []
}

Expected behavior

No TypeScript errors are shown, beautifiers property is allowed.

Error/Stack Traces

[ts]
Argument of type '{ beautifiers: string[]; }' is not assignable to parameter of type 'OptionValues'.
  Object literal may only specify known properties, and 'beautifiers' does not exist in type 'OptionValues'.

System (please complete the following information):

• OS: macOS
• Version: El Capitan, 10.11.6

Additional context

🔥

[Glavin001]

See Excluding certain properties from the index signature in https://basarat.gitbooks.io/typescript/docs/types/index-signatures.html

Related to microsoft/TypeScript#24300 and microsoft/TypeScript#17867

Unfortunately, this is not going to be possible with current version of TypeScript 😢 .

[Glavin001]

After making a bunch of changes, I try it all out:

const test: OptionValues = {
  beautifiers: ["test"],
};

... and it throws an error:

Type '{ beautifiers: string[]; }' is not assignable to type 'OptionValues'.
  Type '{ beautifiers: string[]; }' is not assignable to type '{ [beautifierName: string]: BeautifierSpecificOptionValues & DependenciesForBeautifierRegistry; }'.
    Property 'beautifiers' is incompatible with index signature.
      Type 'string[]' is not assignable to type 'BeautifierSpecificOptionValues & DependenciesForBeautifierRegistry'.
        Type 'string[]' is not assignable to type 'DependenciesForBeautifierRegistry'.
          Index signature is missing in type 'string[]'.ts(2322