Add displayArrayKey property that hides the array keys

[imdanielsp]

Change: Closes #180

This change introduces the displayArrayKey property to the
ReactJsonView component defaulting to true. This property will allow
users to hide the array-key in the component output.

Verification:

• Run unit test
• Added unit test
• Toggle the displayArrayKey and visually verified the key is not
  displayed when expected

[erunion]

@mac-s-g Any chance we can get this in?

[mac-s-g]

this is technically a new feature, so i'll bump the minor patch to 1.21.0, and release to npm.

[nedkelly]

Was this released? I'm using 1.21.1 but I get this type error:

Type '{ src: any; theme: "ashes" | "rjv-default"; indentWidth: number; collapsed: number; collapseStringsAfterLength: number; displayDataTypes: boolean; displayArrayKey: true; style: { fontSize: ReactText; backgroundColor: string; }; }' is not assignable to type 'IntrinsicAttributes & ReactJsonViewProps & { children?: ReactNode; }'.
  Property 'displayArrayKey' does not exist on type 'IntrinsicAttributes & ReactJsonViewProps & { children?: ReactNode; }'.ts(2322)

And displayArrayKey doesn't exist in node_modules/react-json-view/index.d.ts.