regex + wildcard

I should have continued my previous pull request #817, but I'm not how to do it since I used a different fork. 

````R
N=10; 
set.seed(1)
DT <- data.table(
  id =  sample(5, N, TRUE),
  v1 =  sample(5, N, TRUE),                        
  v2 =  sample(5, N, TRUE)                   
)
DT[,.SD, .SDcols = w("v*")]
DT[,.SD, .SDcols = w("v*", "v*")]
DT[,.SD, .SDcols = w("v*", "id")]

DT[,.SD, .SDcols = -w("v*")]
DT[,.SD, .SDcols = -r("^v.*")]
DT[,.SD, .SDcols = r("^v[a-z]")]
DT[,.SD, .SDcols = -r("^v[a-z]")]
````