Make shinyapps.io support pacman usage

[xhdong-umd]

This is done before, but worth to put into issues as we may need to deal with it later.

• Using pacman instead of common library() in code has advantage that it will install packages not available in user's machine. So user can run from github and update packages automatically.
• However pacman is not recognized by shinyapps.io so it will fail in deployment.
• In the backend shinyapps.io used packrat to manage package dependencies. I made some changes to packrat to make it recognize pacman.
• RStudio don't plan to take the change because that means they need to support the feature officially. The ideal solution is for them to have some extension mechanisms.
• Before that, I'm using my changed fork of packrat to deploy to shinyapps.io.