Allow custom Command creation

[billrob]

There is a use case requested to bake in EF Sql Retry logic discussed in #1864. However to do so requires a Property on the IDbCommand to be set. Using only connection.CreateCommand in the CommandDefinition does not allow this to happen.

Please add a new ctor that takes in a Func<IDbConnection, IDbCommand> createCommand delegate and use that if provided in the SetupCommand method.