Add '-l' option to just provide sample names instead of bam files

[winni2k]

I am creating the pileup from read groups in a single file using streaming_pileupy, but this means that I don't have a list of bam files from which to pull the sample names. Creating these files would be expensive. Instead I have list of sample names that I use with streaming_pileupy as well.

Here is an addition to allow sample naming from a simple sample list.