Add request offset to csv

[erwinvaneyk]

This change ensures that the offset of each request is stored (and printed when using csv) to allow users to visualize and analyze temporal relationships between requests.

As a flyby this change fixes a minor bug where the CSV output starts with an empty line.

[rakyll]

It would be good to document the CVS layout on the top of the output. Can you add the column names to the top of the file? Otherwise LGTM.

[erwinvaneyk]

Thanks @rakyll for the quick reply - I added some comments to the top of the file to document the general structure

[rakyll]

Thanks!