Support skipping over groups with the wrong header

[adamnovak]

I'm now wanting to embed some other data into a GAM file. I want to have vg giraffe record its command line options in the GAM without tacking them onto every single Alignment record, or some particular distinguished Alignment record.

The stream format is meant to support this: each group can have a string header at the front (essentially as the first message) denoting the type of the messages in the group, and you can read just the ones you care about.

So I'm wanting to introduce some groups with a header other than "GAM" in the middle of the GAM file. I would want the reader to skip over them, but it looks like right now the reader will see that there's a header different from what it is expecting and bail out.