Since packing can be quite slow, it is nice to display progress to the
caller. We use the excellent github.com/vbauerster/mpb library to do so,
and add a bit of colour with github.com/logrusru/aurora.
Finally, augment the inspector and packer with a summary printer that
displays the overall file size/count, and compression ratio for each
compression type.
If a .svg file doesn't have a “<?xml…” prolog, then it would be detected as plain
text. Allow extension-based override of the content-type in this case.
Laurence Withers