title: arrange_() multiple columns with descending order
body: <p>I am trying to use <code>arrange_()</code> with string input and in one of the columns in descending order.</p>
  
  <pre><code>library(dplyr) # R version 3.3.0 (2016-05-03) , dplyr_0.4.3 
  # data
  set.seed(1)
  df1 &lt;- data.frame(grp = factor(c(1,2,1,2,1)),
                    x = round(runif(5,1,10), 2))
  
  #   grp    x
  # 1   1 3.39
  # 2   2 4.35
  # 3   1 6.16
  # 4   2 9.17
  # 5   1 2.82
  </code></pre>
  
  <p>Below is what I need to achieve:</p>
  
  <pre><code>df1 %&gt;% arrange(grp, -x)
  df1 %&gt;% arrange(grp, desc(x))
  #   grp    x
  # 1   1 6.16
  # 2   1 3.39
  # 3   1 2.82
  # 4   2 9.17
  # 5   2 4.35
  </code></pre>
  
  <p>In my case second column is a string:</p>
  
  <pre><code>#dynamic string
  myCol &lt;- "x"
  
  #failed attempts
  df1 %&gt;% arrange_("grp", desc(myCol))
  </code></pre>
  
  <blockquote>
    <p>Error: incorrect size (1), expecting : 5</p>
  </blockquote>
  
  <pre><code>df1 %&gt;% arrange_("grp", "desc(myCol)")
  </code></pre>
  
  <blockquote>
    <p>Error: object 'myCol' not found</p>
  </blockquote>
  
  <pre><code>df1 %&gt;% arrange_(c("grp", "desc(myCol)"))
  #wrong output
  #   grp    x
  # 1   1 3.39
  # 2   1 6.16
  # 3   1 2.82
  # 4   2 4.35
  # 5   2 9.17
  </code></pre>
  
  <p>I found similar solution <a href="https://stackoverflow.com/questions/26497751">here</a>, but couldn't make it work:</p>
  
  <pre><code>df1 %&gt;% arrange_(.dots = c("grp", "desc(myCol)"))
  </code></pre>
  
  <blockquote>
    <p>Error: object 'myCol' not found</p>
  </blockquote>
  
  <p>Feels like I am missing something very obvious, ideas?</p>