v0.3.1

Patch release — bug fix.

Fixed

• Run now returns the caller's context error (e.g. context.Canceled /
  context.DeadlineExceeded) when the caller cancels the context, instead of
  nil. Previously a cancelled run consumed some or none of the input and
  returned nil, so a truncated run was indistinguishable from a completed one.
  A genuine pipeline error still takes precedence. (#1)