Wildcards

 If .SDcols is an atomic character vector and is not the name of a variable in the data.table,  try to understand .SDcols as a wildcard first instead of throwing an error.

`by` could behave similarly.

```R
N=1e4; K=100
set.seed(1)
DT <- data.table(
  id = 1:N,
  v1 =  sample(5, N, TRUE),                          # int in range [1,5]
  v2 =  sample(1e6, N, TRUE),                        # int in range [1,1e6]
  v3 =  sample(round(runif(100,max=100),4), N, TRUE) # numeric e.g. 23.5749
)
DT[,colMeans(.SD),.SDcols="id v1"]
DT[,colMeans(.SD),.SDcols="id v*"]
DT[,colMeans(.SD),.SDcols=-"v*"]
DT[,colMeans(.SD),.SDcols=-"v?"]
````