title: Avoiding to compile the whole document each time in Rmarkdown
body: <p>I am a new user to R Markdown and was wondering if there was a way to "incrementally" compile an html markdown page as I am writing the code. Say I add 20 lines of code to an existing markdown file today. Is there a way to have the program "remember" the past compilations so that only the 20 new lines are added to the html file preserving the past rendering. I have a lot of memory heavy steps in my code (loading unloading files), and I find that when I add new bits of code I am having to compile everything from the beginning.</p>
  
  <p>I tried looking into the "cache" option but it does not seem to be working.</p>
  
  <p>I am assuming that all the variables I will be needing are present in my environment. In other words, i want to incrementally build an html markdown file without having to compile everything the moment I add an extra line to an existing document. Thanks for your help!</p>