Added highlighter to percolate api.

The highlighter in the percolate api highlights snippets in the document being percolated. If highlighting is enabled then foreach matching query, highlight snippets will be generated.
All highlight options that are supported via the search api are also supported in the percolate api, since the percolate api embeds the same highlighter infrastructure as the search api.
The `size` option is a required option if highlighting is specified in the percolate api, other than that the `highlight`request part can just be placed in the percolate api request body.

Closes #3574