Add more arguments for customization of session logic.

[LTLA]

For example, we can do this to force the Shiny app to close once the user has disconnected:

example(iSEE, ask=FALSE) # create an example
iSEE(sce, customizeSession=function(session) session$onSessionEnded(stopApp))

This is useful for terminating R instances on some custom single-user Shiny deployments, e.g., based on Slurm or cloud services. Other possible uses involve activity logging once a user has finished.

As customizeSession() is run inside the server function, you can even stick your own custom logic in there:

library(shinyalert)

iSEE(sce, customizeSession=function(session) {
     stuff <- new.env()
     stuff$fired <- NA 
     observe({
           invalidateLater(10000, session=session)
           if (is.na(stuff$fired)) {
               stuff$fired <- FALSE # skip first firing
          } else if (!stuff$fired) {
                  shinyalert("FIRING SOME EVENT!")
                  stuff$fired <- TRUE
           }
     }, domain=session)
})

This is a much-simplified version of a timeout alert to alert the user when their session is about to expire.

[kevinrue]

Finally got around to checking this.

Looks nice.

What's the story about customTags? This PR seems to add in in the man page, but without adding it to the function definition nor the R code. Testing me? ;)