Allow access to header.content

```ts
export declare type HeaderUserConfig = Omit & {
readonly content: string;
};
```
Right now it's on readonly mode if you assign the `TableUserConfig` to your config file, however, this prevents reusability if you want to change the content only for another table.
Right now I have to either @ts-ignore the config file input if I want to change the content without assigning a type to it or I have to create a completely new config file.