Update album.py to accept subfolders in config variable folder_format

[cloud74pebble]

Correct handling of subfolders in config variable "folder_format"

e.g.:
[filepaths]
folder_format = "{albumartist}/{title} ({year})"

old method didn't allow creating subfolders by stripping "" or "/"

[nathom]

I think this might cause unexpected behavior if there is a / in any metadata.

[cloud74pebble]

It doesn't affect any metadata.
It is just the storage-path in the filesystem.

with last release it was possible to sort the output to folders:
e.g. Albumartist/Albumtitle
or Albumartist/Year/Albumtitle

Using the current dev-branch it wasn't possible anymore.
This small patch fixed it.

[Delivator]

Bump

[nzoschke]

This works for me too

[nzoschke]

I think this might cause unexpected behavior if there is a / in any metadata.

This looks to be handled properly.

Metadata albumartist, albumcomposer, title all pass through clean_filename before being passed as the format kwargs. So if that metadata has / it is stripped then used as subpaths for the folder_format when it contains /.