additionalScaleFactors -> scaleFactors

This is a simple change, but it's diffed on top of https://github.com/threerings/flump/pull/127, so it looks quite large. If that one gets rejected for whatever reason and we still want this one, I'll spin up a new PR on top of the current master. 

In Flump currently, exporting the scale factor of 1 is required because it's implicit in the list of scale factors. This change (last two commits) changes the configuration property "Additional Scale Factors" to "Scale Factors", making the 1 factor explicit so that if a project doesn't want it exported it can remove it and not pay a runtime penalty to unzip assets it will never use.