title: how to remove NA in summarize_all to summarize mutliple col at the same time?
body: <p>I have a dataframe like this and want to summarize the mean of every col ignoring NA using dplyr:</p>
  
  <pre><code>df= data.frame('var1'=sample(10,3),'var2'=sample(10,3), 'var3'=c(NA, NA,1), 'var4'=c(2,NA,6))
  df %&gt;% summarise_all(mean)
  </code></pre>
  
  <p>however, this will return NA in col 3 and 4.
  How can I pass in na.rm=T?</p>