Add "identity" as a TSV option

[faithokamoto]

I wanted this statistic for myself

Changelog Entry

To be copied to the draft changelog by merger:

• Allow selecting the identity field in vg filter --tsv-out

Description

This statistic is already there (or at least it's there in my files output by vg giraffe) and it's useful, so we might as well expose it via the --tsv-out interface.