todo

necessary
=========

delete halfway-built output files when interrupted

frosting
========

-k flag for "keep going on error".  take a numeric param
for how many errors to swallow before aborting.

when we've been waiting for commands for more than a second
with no output, print some "still waiting for xyz" text

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

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