Create human-readable style identifiers

[cooknl]

The numeric style identifiers for column formatting would be greatly improved with human-readable versions.

In the introductory blog post, you already alluded to this issue by including descriptors:

date_style 6 (the m_day_year style)

time_style 4 (the hm_p style)

Baking this in, and using standard descriptors to do it, i.e. ISO 8601 for dates and times, would be very helpful