Summary option to display total percentage and byte reduction

[jonnyawsom3]

I'm wanting to move away from ECT due to a lack of updates and questionable decisions regarding defaults, however the per-file statistics of oxipng makes for quite a cluttered window and rather hard to see how much of an overall improvement it had. -q stops the flood of file paths, but doesn't display anything at all.

I thought I'd ask about a -S or --summary that would print similar to ECT, allowing for quick comparisons. Something like

Processed 107 files
Could not optimize 12 files further
Saved 192.09MB out of 774.25MB (24.81% Smaller)

[TPS]

This seems complementary to #419, which was solved differently via #488 → #489, but didn't quite do that or this.

[andrews05]

Hi @jonnyawsom3
This could be a useful feature to have. Unfortunately right now, both this and the json output proposed in #583 require a breaking change to the API before they can be implemented*. So it won't happen before v10.

*Specifically, the optimize function will need to return the input and output sizes so that main.rs can aggregate them.