Extend the "Payment Term" Options

At this time the only payment terms supported are

xDays -> x days after invoice date

xEOM -> x days after end of month that the invoice was issued

 where x = number of days

It would be great if WorkGuru could extend to also support

Last day of next month (LDNM)

z day of next month (zDNM)

 where z = day of the month

Ideally if be even better if could define the payment term with our own description along with a method-name and number-of-days

e.g. 

"Prepayment", Number of days, 0

"7 days from invoice", Number of days, 7

"10th day of next month", Day of next month, 10

"14 days from end of month", Days since the end of the month, 14

"Last day of next month", Last day of next month, n/a