Add pattern-matching variable substitutions

[o-smirnov]

I'd like to be able to refer to "the most recent step of a particular kind". See e.g. https://github.com/o-smirnov/jovial/blob/master/ddf-mosaic.yaml. Here, a mask is made at step mask-di2, and later reused and reused (see e.g. image-dd1). If I decide to make another mask at a later step (say mask-di3), I now have to go and edit all the subsequent references to use that mask.

It would be nice to be able to say {steps.mask-*.foo}, and have the middle bit interpreted as "the last (or at least alphanumerically highest) preceding step matching mask-*". This should be easy to add to substitutions.py.