[Proposal] Support search rankings

[ColonelBundy]

The ts_rank and ts_rank_cd methods are really useful when creating a search.
A proposal to support this:

public enum RankType
{
    Frequency, // ts_rank
    Density // ts_rank_cd
}

public enum RankOrder
{
    Descending, 
    Ascending
}

public class SearchOptions<T>
{
    public RankType RankType { get; set; } = RankType.Density;

    public RankOrder OrderBy { get; set; }

    public Expression<Func<double, bool>> Where { get; set; } // only display rank over/equal to X

    public Expression<Func<T, double>> MapRanking { get; set; } // output ranking to this property
}

using (var session = Store.LightweightSession())
{
    session.Query<Document>().Where(x => x.PlainTextSearch("Some title", new SearchOptions<Document>
    {
        MapRanking = x => x.Rank, // Optionally output rank to this property on Document
        RankType = RankType.Frequency, // we wish to use ts_rank instead of ts_rank_cd
        Where = x => x >= 1.0, // Optionally only return results with ranking over or equal to 1.0
        OrderBy  = RankOrder.Ascending // Optionally order by the rank property if you can't output it to a property.
    }));
}

MapRanking only exists because there may be instances where you'd want to display it and instances where you want to and wish to order by the rank, then you could use OrderBy.

More info about ts_rank here

[oskardudycz]

@ColonelBundy that looks good to me.
Would you be willing to provide PR for that? I can try to provide some guidance.