Default the consumer library to read from latest

Having consumers start at latest record seems like a reasonable default.
This can also be overridden with optional config, make sure that is
documented for users of the library.

Fixes: https://github.com/harlow/kinesis-consumer/issues/86

README.md

@@ -234,6 +234,22 @@ The [expvar package](https://golang.org/pkg/expvar/) will display consumer count
 },
 ```
 
+### Consumer starting point
+
+Kinesis allows consumers to specify where on the stream they'd like to start consuming from. The default in this library is `LATEST` (Start reading just after the most recent record in the shard).
+
+This can be adjusted by using the `WithShardIteratorType` option in the library:
+
+```go
+// override starting place on stream to use TRIM_HORIZON
+c, err := consumer.New(
+  *stream,
+  consumer.WithShardIteratorType(kinesis.ShardIteratorTypeTrimHorizon)
+)
+```
+
+[See AWS Docs for more options.](https://docs.aws.amazon.com/kinesis/latest/APIReference/API_GetShardIterator.html)
+
 ### Logging
 
 Logging supports the basic built-in logging library or use thrid party external one, so long as

consumer.go

@@ -26,7 +26,7 @@ func New(streamName string, opts ...Option) (*Consumer, error) {
 	// new consumer with no-op checkpoint, counter, and logger
 	c := &Consumer{
 		streamName:               streamName,
-		initialShardIteratorType: kinesis.ShardIteratorTypeTrimHorizon,
+		initialShardIteratorType: kinesis.ShardIteratorTypeLatest,
 		checkpoint:               &noopCheckpoint{},
 		counter:                  &noopCounter{},
 		logger: &noopLogger{