Let the user add a custom comment to the import log

I know the log file already distinguishes nicely between situations in which autotagging was not applied by using terms as `duplicate-skip`, `skip` and `asis`, which is really nice!

I would be even happier if in the case of an interactive import session in which I choose 'Use as is', I'd be able to add a comment, because often there's some work to be done with this imported data and sometimes writing some of the specifics down is handy. This way, when I look for occurences of `asis` in the log file, I immediately know what needs to be done.

Personally, I'd say the ideal workflow would be to choose 'Use as is' by pressing `u`, after which the importer prompts for a comment, which you can leave empty by pressing `RETURN` immediately.

Thank you for this amazing tool!