necessary
=========

website, contact info

-j flag for parallelism

delete halfway-built output files when interrupted

frosting
========

if command line for an output changed, no need to even stat
the output, just mark it for rebuilding immediately.

"explain" mode that shows why we're building a file

"one file" mode that builds only outputs of a single file

adjust to system load dynamically

progress bar rather than printing commands in series

compute etas on builds using logged timing info
  how does parallelization fit in?