title: Issue Building R package with RcppArmadillo
body: <p>I am building an R package with RcppArmadillo using RcppArmadillo.package.skeleton() in order to facilitate usage.</p>
  
  <p>My code is structured as follows:</p>
  
  <pre><code>RcppArmadillo.package.skeleton(name = "HACSim", code_files = c("accumulate.cpp", "HAC.sim.R", "HAC.simrep.R"),example_code = FALSE).
  </code></pre>
  
  <p>It runs fine. However, I get a warning message:</p>
  
  <pre><code>Warning message:
  In (function (name = "anRpackage", list = character(), environment = .GlobalEnv,  :
    Invalid file name(s) for R code in ./HACSim/R:
    'accumulate.cpp'
   are now renamed to 'z&lt;name&gt;.R'
  </code></pre>
  
  <p>I find it odd that R does not recognize the .cpp extension.</p>
  
  <p>Is this not the correct way of specifying a C++ file using RcppArmadillo.package.skeleton()?</p>