Pass in variable from user???

Using PL/PGSQL, I am trying to create a procedure to display the count
of rows in any single table of a database. The End-user would pass in a
table name and the prodecure would display the table name with the row
count.
I am able to hardcode the variable for table and get the appropriate
results from my count function (see below), but cannot pass in a
variable and have the function work. Any suggesstions???

CREATE FUNCTION get_table_count(tablename text) RETURNS integer AS $$
  DECLARE

    --tablename ALIAS FOR $1;

     rowcount INTEGER;
  BEGIN

    SELECT INTO rowcount count(*) FROM tablename;

    RETURN rowcount;

  END;
$$ LANGUAGE 'plpgsql';