Use an array as the default value so that (a) we don't point at directories that don't exist and (b) we hint to the client programmer that more than one directory can be added. This way you can add more than one plugins dir, too, giving you more flexibility.
It can be useful to get a variable accessible in all views using a global context (e.g. Twig Macros)
… other drivers and in a non-configurable way. Made it a lib-specific config setting.