Accomodate data.table sources including iDate type

Prework

• Read and abide by pointblank's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

Fantastic package! As requested in some other isues, I would like to validate my data in data.tables. Currently the draft_validation fails on the IDate type

R> DT <- data.table(date=as.Date(c("2016-05-26", "2016-04-20", "2016-06-01", "2015-06-05", "2015-06-05")),var=seq(1:5))
R> pointblank::draft_validation(DT,overwrite = T)
✓ The pointblank .R file has been written to `/home/cfljam/DT.R`
R> DT <- data.table(date=as.IDate(c("2016-05-26", "2016-04-20", "2016-06-01", "2015-06-05", "2015-06-05")),var=seq(1:5))
R> pointblank::draft_validation(DT,overwrite = T)
Error in if (column_role == "integer.discrete") { : 
  missing value where TRUE/FALSE needed