specify which step to combine in create_task_makefile

[aappling-usgs]

We may want to support another argument to create_task_makefile that allows you to specify which step you want each combiner to apply to. This would apply to the use case where, for example, step 2 creates a data.frame and step 3 creates a png, and combiner 1 builds a big data frame from all the step 2 data.frames while combiner 2 builds a big gif from all the step 3 pngs.