support for --tmpdir DIR https://github.com/Factual/drake/issues/83
When adding a new non-flag arg to Drake, we also have to update the first let statement core/split-command-line. (This is related to Drake's ability to understand the difference between options and step selection syntax)
Merge pull request #84 from guillaume/tmpdir

support for --tmpdir DIR https://github.com/Factual/drake/issues/83