title: Join with non R standard column name in dplyr pipeline
body: <p>I know how to join like so:</p>
  
  <pre><code>library(dplyr)
  
  d1 &lt;- data_frame(
    x = letters[1:3],
    y = LETTERS[1:3],
    a = rnorm(3)
    )
  
  d2 &lt;- data_frame(
    x2 = letters[3:1],
    y2 = LETTERS[3:1],
    b = rnorm(3)
    )
  
  left_join(d1, d2, by = c("x" = "x2", "y" = "y2"))
  </code></pre>
  
  <p>What can I do when column y2 is called:</p>
  
  <pre><code>bla y2
  </code></pre>
  
  <p>I tried various approaches without success:</p>
  
  <pre><code>left_join(d1, d2, by = c("x" = "x2", "y" = "bla y2"))
  left_join(d1, d2, by = c("x" = "x2", "y" = "`bla y2`"))
  left_join(d1, d2, by = c("x" = "x2", "y" = `bla y2`))
  </code></pre>