Run to parameter

Given a list like:

    list( import = list(file = ..., ...),
          data = list("Step one" = list(...),
                           "Step two" = list(...), ...),
          model = list(...), export = list(...), ...)

we can now do:

`stagerunner$run('import', to = 'data/two')`

and it will perform all the steps from `import` to `data/Step two`, inclusive.