Delete all records NOT referenced by Foreign Keys

I'd like to run a clean up command on my tables to
eliminate rows that I'm no longer using in the database.

I want to do something like this:

DELETE FROM tablename
WHERE IS_REFERENCED_BY_FOREIGN_KEY IS FALSE;

Does anyone know how something like this could be done
in PostgreSQL? I know I can search all the tables that
I know refer to this table and see if my primary key
exists, but I want a solution that does not require me to
rewrite my code every time a new foreign key constraint
is added to the database.

There must be a way to ask PostgreSQL for a reference count
on a given row or something.

Thanks,

Dante

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D. Dante Lorenso
dante(at)lorenso(dot)com