Support multiple image directories

[dannyadair]

images refers to filenames inside images_dir which defaults to the relative path "images".
That should just be one special case used by default.

Support configuration of multiple image directories, where a particular image has to specify which of the directories it is from. (support names for easier reference)

EDIT: Bring in pydantic first as per #26

[dannyadair]

Will not make it in this time. But next!

• TODO: allow directories.images to be either a PathSpec or a list of Pathspecs
  an image then can refer to its directory by the same name
• TODO: change examples from images_dir to use directories.images (breaking change)

[dannyadair]

While at, it simplify the settings of an image. You can set anything you like if you use it yourself - settings should only be prescribed for things that are then actually used by pdfbaker. We're setting "type" to "default" if not given, but not actually use it...