title: dplyr - groupby on multiple columns using variable names
body: <p>I am working with R Shiny for some exploratory data analysis. I have two checkbox inputs that contain only the user-selected options. The first checkbox input contains only the categorical variables; the second checkbox contains only numeric variables. Next, I apply a <code>groupby</code> on these two selections:</p>
  
  <pre><code>var1 &lt;- input$variable1      # Checkbox with categorical variables
  var2 &lt;- input$variable2      # Checkbox with numerical variables
  
  v$data &lt;- dataset %&gt;%
    group_by_(var1) %&gt;%
    summarize_(Sum = interp(~sum(x), x = as.name(var2))) %&gt;%
    arrange(desc(Sum))
  </code></pre>
  
  <p>When only one categorical variable is selected, this <code>groupby</code> works perfectly. When multiple categorical variables are chosen, this <code>groupby</code> returns an array with column names. How do I pass this array of column names to <code>dplyr</code>'s <code>groupby</code>? </p>