Add bias-trimming, need to think more about how this works with strand

opening this for feedback. does the trimming need to consider strand? E.g.:

```C
if (strand == 1 || strand == 3){
    if(pos <= config->trim5) return 0;
    if(pos > (plp->b->core.l_qseq - config->trim3)) return 0;
} else {
    if(pos <= config->trim3) return 0;
    if(pos > (plp->b->core.l_qseq - config->trim5)) return 0;
}
```