transform_row >>> format_timestamp for semantics

[nickymikail]

Saying what the pre_hook does should make it easier for people first learning the technology.

[cmerrick]

Hi @nickymikail, thanks for your contribution!

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[briansloane]

@nickymikail This change looks great. Can you please sign the Singer CLA so that we can merge?

https://stitch-cla-enforcer.herokuapp.com/

[nick-mccoy]

This PR will be covered in #7