This runs sprintf in two passes: once to apply the escape sequences and
again to interpolate variables.  This requires `%%` for the second pass,
which _can_ be confusing.

Maybe this approach is too much for the very simple highlighting in this
version?  But, it seems to work okay for elaborate color schemes, too.
IMO it's easier to read and maintain than a bunch of conditional string
appending.  And it's simpler than the other templating approaches that I
considered.