title: full_join by variable as column names
body: <p>I'm trying to use dplyr's <code>full_join</code> to combine two data.frames, for example: </p>
  
  <pre><code>col1 = 'b'
  col2 = 'd'
  
  df1 = data.frame(a = 1:3, b = 1:3)
  df2 = data.frame(a = 1:3, d = 1:3)
  
  
  full_join(df1, df2, c('a' = 'a', col1 = col2))
  </code></pre>
  
  <p>but it returns </p>
  
  <blockquote>
    <p>Error: <code>by</code> can't contain join column <code>col1</code> which is missing from LHS</p>
  </blockquote>
  
  <p>I'm looking for an output similar to</p>
  
  <pre><code>merge(df1, df2, by.x = c('a', col1), by.y = c('a', col2))
    a b
  1 1 1
  2 2 2
  3 3 3
  </code></pre>