Add support for highlighting and context

[Ceres6]

Is your feature request related to a problem? Please describe.
In some context when you are showing results of a query, especially for large documents, it would be useful to have the context where the matching occurred inside the document, to help the user to choose between different results.

Describe the solution you'd like
It should be possible to specify a context length and get an array with data on where matches occurred and tokens/word around that.

My Idea would be to add a field to results like

{
  context: {
    match: string
    position: {
      start: number
      end: number
    }
  }
}

Describe alternatives you've considered
Really any way to get similar info with different format, maybe a snippet.

Additional context

[micheleriva]

As discussed separately, this will start as a plugin. Thanks Carlos!

[n4bb12]

It would be great if highlights could be returned in a variant that already contains <mark> wrappers. That way, folks wouldn't need to consume the positions manually and implement something themselves. See Algolia's output for comparison.