The graphiql property doesn't allow assigning any value with type other than a boolean (TypeScript).
- Typescript: 3.9.3
- koa-graphql: 0.8.0
- @types/koa-graphql: 0.8.3

I'm tried to set up the "Darkula" theme for GraphiQL following this manual: [Custom GraphiQL themes](https://github.com/graphql-community/koa-graphql#custom-graphiql-themes), but get an error about a type.

`Type '{ editorTheme: string; }' is not assignable to type 'boolean'.`

**Code**
```ts
graphqlHTTP({
    schema,
    graphiql: {
        editorTheme: 'darkula',
    },
})
```
**Type definitions**
```ts
    /**
     * A boolean to optionally enable GraphiQL mode.
     */
    graphiql?: boolean;
```