I needed this for myself, so I spiffed it up a bit and now you can have your own copy too.
Many thanks to ~OkazakiTomoya97
with some of the translations.
If you need more info on using this plug-in, there's a tutorial in *TehPuroisen
's gallery: fav.me/d4w8hiv
Thanks to ~OkazakiTomoya97 for pointing this out:
The extra shoulder bone isn't shown in the picture because it's in the same spot as the original shoulder bone. The extra one will be named left to right 'ShoulderP_L' and 'ShoulderP_R' respectively, double click to select the bone.