Add an ATAC-seq mode

[charles-plessy]

This mode will:

• Add 4 to start positions and subtract 5 to the end positions, so that neighbour tagments do not overlap,
• have a single bloc, since there is no splicing, and
• indicate the centre of the interval as a thick region.

Please do not hesitate to submit a pull request if you are faster than me. I can not start to work on this until October.