title: double quotes escaping in golang exec
body: <p>i need to run following command:</p>
  
  <pre><code>ffmpeg -i input.jpg -vf scale="'if(gt(a,4/3),320,-1)':'if(gt(a,4/3),-1,240)'" output_320x240_boxed.png
  </code></pre>
  
  <p>so i execute: </p>
  
  <pre><code>cmd = exec.Command("ffmpeg", "-i", "input.jpg", "-vf", "scale=\"'if(gt(a,4/3),640,-1)':'if(gt(a,4/3),-1,300)'\"", "output_320x240_boxed.png")
  </code></pre>
  
  <p>it fails with following error:</p>
  
  <pre><code>Error when evaluating the expression 'if(gt(a,4/3),-1,300)"'.
  Maybe the expression for out_w:'"if(gt(a,4/3),640,-1)' or for out_h:'if(gt(a,4/3),-1,300)"' is self-referencing.
  </code></pre>
  
  <p>Command works when executed in command line.
  Why does it happen and how can i escape those double quotes to prevent this error?</p>