More serialization configuration

[carl-berg]

Enables configuration of CosmosClient in order to fully configure serialization (until #40 is solved).
This PR enables a user to configure the same serialization for both DocumentClient and CosmosClient which is a bit cumbersome, but at least possible. Example:

public static DatabaseOptions<MyDatbase> BuildDatabaseOptions(string connectionString)
{
    var builder = new DatabaseOptionsBuilder<MyDatbase>().UseConnectionString(connectionString);
    builder.Options.DatabaseId = "MyDb";
    builder.Options.ConfigureJsonSerializerSettings = settings =>
    {
        settings.Converters.Add(new CosmosDbJsonSerialization.CustomJsonConverter());
        builder.Options.ConfigureCosmosClientOptions = options =>
        {
            options.Serializer = new CosmosDbJsonSerialization.CosmosJsonNetSerializer(settings);
        };
    };

    return builder.Options;
}

An example of a CosmosJsonNetSerializer can be found here: https://gist.github.com/richstokoe/c82fc831c6b4020926f5b5f772f7cd70