grep chrom name in the first BED column only

[darked89]

Stricter grep so chromosome names without "chr" prefix can be used. Also a bit faster. Minimal code change. Tested

This is a stopgap solution since grepping through tens of millions rows for every chromosome can be replaced by i.e:

1. requiring that the input BAM/BED files are sorted by position
2. switching from grep to xsv
3. indexing BED files with xsv
4. retrieving the rows for a given chromosome, required columns only

Above makes sense if you want to introduce just the minimal code changes:

1. only xsv will need to be installed
2. BED file remains as the principal input