title: specify dplyr column names
body: <p>How can I pass column names to dplyr if I do not know the column name, but want to specify it through a variable?</p>
  
  <p>e.g. this works: </p>
  
  <pre><code>require(dplyr)
  df &lt;- as.data.frame(matrix(seq(1:9),ncol=3,nrow=3))
  df$group &lt;- c("A","B","A")
  gdf &lt;- df %.% group_by(group) %.% summarise(m1 =mean(V1),m2 =mean(V2),m3 =mean(V3))
  </code></pre>
  
  <p>But this does not</p>
  
  <pre><code>require(dplyr)
  someColumn = "group"
  df &lt;- as.data.frame(matrix(seq(1:9),ncol=3,nrow=3))
  df$group &lt;- c("A","B","A")
  gdf &lt;- df %.% group_by(someColumn) %.% summarise(m1 =mean(V1),m2 =mean(V2),m3 =mean(V3))
  </code></pre>