Allow --trace to optionally show UTF-8 unencoded

Users in some locales are at a disadvantage as they cannot read ``` [trace] Parsed [Para [RawInline (Format "html") "<!-- \21487\24796\26412\31 at line 621 [trace] Parsed [Para [Str "\27491\22914\12300\38272\29260\25033\25505\20160 at line 625 ``` easily. Therefore I propose `--trace` also optionally allow passing UTF-8 unencoded, perhaps via `--trace=raw` or `--trace=utf-8`. Or maybe it should be the default, and there should be a `--trace=ascii` if people need it. As you see there are split `\31` characters too, so elaborate decoders are needed.