Consider a mechanism for preloading (and pre-parsing?) files

This pops out to me in particular in the kerning work in `ufo2fontir`; first we need to load/parse the `groups.plist` from the default master, and then in a loop for each master we load and parse the `kerning.plist` before we process the values. It would be nice if we could start loading the things that we know we'll need right when execution begins.

In a world where our context is more general, we could maybe even treat these things like any other sort of resource.

related to #639; it would also be nice if we could start running per-master as the files were loaded.