Allow strong_rec_thresh = 0 for only importing perfect 100% matches

While a working on my already curated library, I set `strong_rec_thresh = 0` and run `beet im -q` to only import 100% perfect albums (so that I can then look at the rest), but it seems that acts as if `-t` was specified.

I didn't look at the code, but I guess there's something `match_accuracy < strong_rec_thresh`?

If that's the case, then IMHO it should check with `<=` (if one says "the threshold for strong recommendations is 5%", i expect 95% matches and up to be strong recommendations). In case you are concerned about the backwards-compatibility of such change, then handling `strong_rec_thresh = 0` would IMHO still be worth it.

BTW: if anyone stumbles upon here because they have the same issue, something like `strong_rec_thresh = 0.0001` seems to be a valid workaround.