Allow string options$columnDefs$targets

[shrektan]

Due to implentation of DT, options$columnDefs$targets doesn't support string values (See #512).

It's inconvinient as using column-names is always easier. In addition, it confuses the user in the first place, as it is inconsistent with DataTables' documentation.

This PR addresses this issue by converting the string valued targets into column index values of the data, internally.

CODE

x <- rnorm(10)
rk <- rank(x)
prefix <- sample(LETTERS[1:3], replace = TRUE, 10)
x_txt <- sprintf("%s%0.2f%%", prefix, x * 100)
tbl <- data.frame(V1 = x, V2 = x_txt, V3 = x, RK = rk)
DT::datatable(tbl, options = list(
  scrollY=FALSE, 
  columnDefs = list(
    list(visible = FALSE, targets = list("V3", 2))
  )
))

Before the PR ("V3" is visible but it should not)

After the PR ( "V3" is now invisible, as expected)

[yihui]

Sounds great! Feel free to merge the PR by yourself whenever you are ready (I only had a quick look). Thanks!