Allow rsgain easy to take multiple directories

[LunarLambda]

I sort my library as Artist/Album, however for singles I just place them in Artist/Song.flac. This makes using rsgain easy a little more involved: I currently run it in a loop over individual album directories, like so:

for dir in */*(/); do rsgain easy $dir; done

I could shorten it to just rsgain easy */*(/) if easy mode allowed passing multiple directories.

For singles I then run rsgain custom -S -s i -l -18 -c p -m 0 */*.flac.