Add upsert functionality

[stefanw]

Common enough use case I believe.

Would get rid of code like

if not table.update(...):
    table.insert(...)

[gka]

Well that's what we already have since version 0.0.1, I think. See
https://dataset.readthedocs.org/en/latest/api.html#dataset.Table.upsert

[stefanw]

Sweet. Sorry, missed it in the docs. Maybe put it in the Quickstart part.

[gka]

come on, it's praised as the second big feature on the homepage: https://dataset.readthedocs.org/en/latest/#features :)

(and it has been there even since sqlaload times, see https://github.com/okfn/sqlaload#sqlalchemy-loading-tools )

(( which pudo authored a year ago okfn/sqlaload@8a5581d :-) ))