title: Does install_github recursively install dependencies?
body: <p>The R package <code>devtools</code> seems the method of choice for installing R packages directly from their github repository, using the <code>install_github()</code> function. </p>
  
  <p>While doing so, not only the addressed github-package is installed, but also its direct CRAN-dependencies, i.e., the Depends and Imports as defined in the Description/Namespace files of the package. </p>
  
  <p>However, I am wondering if indirectly required packages such as those in turn required by the Depends and Imports packages become automatically installed along, too. </p>
  
  <p>For example, when installing the <code>DT</code> package via <code>install_github("rstudio/DT")</code>, the Imports package <code>htmltools</code> is also installed (set <code>dependencies=TRUE</code>), but not the <code>Rcpp</code> package, on which the former depends (R 3.3.1, Rtools34, Win10, x64). Do such packages need to be installed in another way? </p>