Type converters

Support for registering type converters to support scenarios like the following:

```c#
// The model
[Serializable]
public class Activity
{
    public Secured<int> Id { get; set; }
    public string Name { get; set; }
}

// The converter
public class SecuredIntDapperConverter : IDapperConverter
{
    public object Convert(object source)
    {
        if (source is int)
            return new Secured<int>((int)source);
            
        return source;
    }
}

// The converter registration
DapperConverters.Add(typeof(Secured<int>), new SecuredIntDapperConverter());

// The query
var results = connection.Query<Activity>("Activity_All", commandType: CommandType.StoredProcedure);
```