Handling missing value when group contain missing values

[elong0527]

Code below will report error if there is missing values in the ARM variable.

library(metalite.table1)
metalite_table1(~ AGE + SEX + RACE + BMIBLGR1 | ARM, data = r2rtf::r2rtf_adsl, id = "USUBJID")

We need an argument missing_group to handle:

1. `missing_group = "ignore": ignore missing value in group with proper message to user.
2. allow user specify missing as a separate group, the option should allow
   - missing_group = "count": Missing group should be counted into the total column and placed before "Total" column
   - missing_group = "display": Missing group should not be counted into the total column and placed after "Total" column