Add argument for default MM/DD to add to just YYYY inputs about anytime

[eddelbuettel]

I answered related questions a few time on StackOverflow. In short, YYYY-MM is not a date as a date also needs a day. As such, some of the underlying parsers will always fail. Some parsers are sometimes generous and fill in:

> library(anytime)
> anydate("2020")      ## Boost parser fills in 01 and 01
[1] "2020-01-01"
> 
> anydate("2020", useR=TRUE)        ## R parser refuses
[1] NA
> 

I think your best bet here will be preprocessing.  This is outside the scope of `anytime`.