* selectInput/selectizeInput: Fix handling of character(1) choices
* Re-document
* Add .github to .Rbuildignore
* Expand comment, don't import stats
* Add test for 013-selectize regression
* Expand comment
* Split listify into series of passes
* Thouroughly overhaul and comment choicesWithNames()
* No recursion
* Comment new "flat" choice processing machinery
* Remove unneccesary test of choice tree with depth > 2
* Test for choices idiomatically
* Tweak comment for asCharacter
* Comment odd test, add a new test for single-item list
* Handle empty non-lists correctly, add test
* Add test ensuring empty lists come back named
* Add comment/assertion stipulating processGroupedChoices() takes only lists
* allow the `choices` argument in `checkboxGroupInput()` to be `NULL` or `c()` to keep backward compatibility with Shiny < 1.0.1 (fixes#1649)
* use vapply
* added one more test; reimplemented logic for checking if choice args are null
