Add shard iterator type as config option

Fixes: https://github.com/harlow/kinesis-consumer/issues/74

consumer.go

@@ -47,6 +47,13 @@ func WithClient(client kinesisiface.KinesisAPI) Option {
 	}
 }
 
+// ShardIteratorType overrides the starting point for the consumer
+func WithShardIteratorType(t string) Option {
+	return func(c *Consumer) {
+		c.initialShardIteratorType = t
+	}
+}
+
 // ScanStatus signals the consumer if we should continue scanning for next record
 // and whether to checkpoint.
 type ScanStatus struct {
@@ -65,6 +72,7 @@ func New(streamName string, opts ...Option) (*Consumer, error) {
 	// new consumer with no-op checkpoint, counter, and logger
 	c := &Consumer{
 		streamName:               streamName,
+		initialShardIteratorType: "TRIM_HORIZON",
 		checkpoint:               &noopCheckpoint{},
 		counter:                  &noopCounter{},
 		logger: &noopLogger{
@@ -92,6 +100,7 @@ func New(streamName string, opts ...Option) (*Consumer, error) {
 // Consumer wraps the interaction with the Kinesis stream
 type Consumer struct {
 	streamName               string
+	initialShardIteratorType string
 	client                   kinesisiface.KinesisAPI
 	logger                   Logger
 	checkpoint               Checkpoint
@@ -258,7 +267,7 @@ func (c *Consumer) getShardIterator(streamName, shardID, lastSeqNum string) (*st
 		params.ShardIteratorType = aws.String("AFTER_SEQUENCE_NUMBER")
 		params.StartingSequenceNumber = aws.String(lastSeqNum)
 	} else {
-		params.ShardIteratorType = aws.String("TRIM_HORIZON")
+		params.ShardIteratorType = aws.String(c.initialShardIteratorType)
 	}
 
 	resp, err := c.client.GetShardIterator(params)