REMIND: Adds some birthdays and a years function

The years function takes a year and returns the number of years since
then. This is currently used so Remind can tell me how old people are on
their birthdays but could also be used for anniversaries.

remind/.local/share/remind/functions.rem

@@ -0,0 +1,3 @@
+# Function to calculate how many years since a date
+# Useful for birthdays, anniversaries etc.
+FSET years(y) ($Uy-y)