Make `selected` work for server-side selectize input

@jcheng5 `updateSelectizeInput(..., selected = ?, server = TRUE)` will add `selected` as options for selectize

I mentioned loading the first n (n = `maxOptions`) options at initialization, but that does not seem to be possible since selectize does not have an api to start searching (with an empty string), and the only way for searching seems to be typing in the box.