[generic] better directory formatting/md

[allendema]

Adds better fields which can be used in the config like path (url path) or subcategory (domain).
Note: Changes the default directory_fmt to use those.

Before:

./gallery-dl/generic/https:__www.nongnu.org_lzip_/lzip.png

After:

./gallery-dl/generic/www.nongnu.org/lzip/lzip.png