The rmdexamples package illustrates various techniques for extending R Markdown. The examples included here are described in more detail in the article on Shiny Widgets.
To try out rmdexamples locally, you should first install the devtools package, then:
if (!require("devtools")) install.packages("devtools")
# you may also need to update packages: update.packages(ask = FALSE)
devtools::install_github("rstudio/rmdexamples")