Unimported plugin - new option to add globbing

[Mikaciu]

Description

Add a new option, ignore_as_globs to consider the ignore_subdirectories as globs, the same way the ignore section works.

Using this flag, the unimported plugin is walking through the library using the util.sorted_walk method, the same way the importer does.

To Do

• I didn't find a way to access the beets config from a plugin (maybe due to the fact that plugin config is namespaced). An improvement would have been to use the ignore config value instead of duplicating it.
• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.) <-- will be done after review ;)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.