feature request: many factor select support server

This package is great!

When there are many options, such as more than 20000, provide server support.
for example:

Application scenario: For example, if there are some important content, such as gene names, I want to provide a drop-down menu search instead of a text box.
```R
library(shiny)

ui <- fluidPage(
selectizeInput("test",label = NULL,
choices = NULL)
)

server <- function(input, output, session) {
updateSelectizeInput(inputId = "test",choices = paste0("Gene",1:50000),server = TRUE)
}

shinyApp(ui, server)
```