It would be great to have a -quiet option that suppressed logging to stderr except for errors.
I'm using mapshaper on the command line as part of a pipeline that processes tens of thousands of files. It works great! However since mapshaper prints the names of all the files it creates, I have to choose between an overloaded logging window or redirecting stderr to /dev/null. Then I miss any errors (which are my fault, mapshaper is great).
Good suggestion, I'll plan to add -quiet for the next release.