Added the monthly functionality to the recurring transaction generator.  Now can generate a transaction every n months, on a list of days. e.g. 1st and 15th of every month, or 5th of every 3rd month (aka quarterly)
Have not tested trying to select a day number greater than the number of days in a month (e.g. every 30th in a span that includes February), would probably fail.  Probably need to allow a "last day" option.