add config option for supporting reads cutoff

currently, the number 10 is used [here](https://github.com/aryarm/as_analysis/blob/afe62624e004742210f951ad3910893b55805b74/scripts/prepare_counts-rna.r#L57) and [here](https://github.com/aryarm/as_analysis/blob/afe62624e004742210f951ad3910893b55805b74/scripts/prepare_counts.r#L62)

but it might be nice to allow the user to choose