Option to keep all feature properties

[springmeyer]

https://github.com/mapbox/vtshaver/pull/20 landed the ability to have vtshaver automatically remove feature properties that are not needed for a given style.

However there are use cases (like querying) where some users might want feature properties to be kept.

A sophisticated solution for this would be to have vtshaver respond to metadata in the style that declares which properties to keep on which layers. However a more near-term solution I think would be to open up an option, exposed in the vtshaver CLI tool, that would disable all property shaving.