title: ignore NA in dplyr row sum
body: <p>is there an elegant way to handle NA as 0 (na.rm = TRUE) in dplyr?</p>
  <pre><code>data &lt;- data.frame(a=c(1,2,3,4), b=c(4,NA,5,6), c=c(7,8,9,NA))
  
  data %&gt;% mutate(sum = a + b + c)
  
  a  b  c sum
  1  4  7  12
  2 NA  8  NA
  3  5  9  17
  4  6 NA  NA
  </code></pre>
  <p>but I like to get</p>
  <pre><code>a  b  c sum
  1  4  7  12
  2 NA  8  10
  3  5  9  17
  4  6 NA  10
  </code></pre>
  <p>even if I know that this is not the desired result in many other cases</p>