Extensibiltiy: Custom column types

[gentledepp]

We want to use spatialite with sqlite-net.

one thing we needed to implement was the ability to load extensions see here for the branch

then, running raw queries and returning the results as text worked.

However, we also wanted to be able to map NetTopologySuite types.

That way we could define a table like:

[Table("buildings")]
public class Building
{
    [PrimaryKey]
    [AutoIncrement]
    public long Id { get; set; }

    public string Name { get; set; }

    public string Address { get; set; }

    public GeometryCollection Geometry { get; set; }
}

So we need some extension mechanism, since building NetTopologySuite right into sqlite would not make sense.

Stay tuned for the PR...