Use Artist Credits for tag data, but "actual" artist data for filenaming

Currently beets will always normalise [artist credit](https://musicbrainz.org/doc/Artist_Credit) data to the current artist name. However, I want to see when playing the music when, e.g., [Orgi-E](https://musicbrainz.org/artist/345fe3da-b2cb-4ad4-a1a5-43afc903663d) was credited as [Klamfyr](https://musicbrainz.org/release/d09b3568-e9cc-4458-bcf7-0c215cca75ce), but I still like the normalisation for file tree organisation purposes. This should probably be an option though, as other people will likely want to always normalise the name (and others might want to not normalise the name in the path as well).

(Somewhat related morituri issues: thomasvs/morituri#80, thomasvs/morituri#48)