Allow "dropping" transcripts with no gene from gene-level output

[rob-p]

Currently, the default (and only) behavior of Salmon when aggregating from the transcript to gene level is to return transcripts with no corresponding gene in the GTF or 2-column format file as their own gene. In some cases, it would be desirable, instead, to simply filter out or "drop" such transcripts from the gene-level output. We should add a flag to optionally enable this behavior.

Feature requested by @demis001.