Make deleting directories more convenient

[TadLeonard]

Per @hntuan94, it would be convenient if tfatool exposed a delete_directory function. This function would:

• walk through the directory and delete any files within
• when the directory's empty, it'd delete the directory itself

The challenge here is to be absolutely sure it's empty before deleting. Listing the contents of the directory before making the final delete would be a good idea. Care is needed because FlashAir will let you delete a directory that isn't empty, but the files within will become unreadable and undeletable forever afterwards. Scary!

[TadLeonard]

Alternatively, the function could play it safe and:

• check whether the directory's empty, raise an exception if it's not
• delete the directory

We could then expose a separate function for deleting a list of files, whose usage pattern would look like:

• files_to_delete = command.list_files("directory", session)
• upload.delete_files(files_to_delete, session)
• upload.delete_directory("directory", session)