title: Change (all) facet_grid strip text
body: <p>How can I change the (all) label given by <code>facet_grid</code> to the strip text if <code>margins = ...</code>?</p>
  
  <p>Here is an example:</p>
  
  <pre><code>ggplot(mtcars, aes(mpg, wt)) + geom_point() +
      facet_grid(am ~ cyl, margins = "cyl")
  </code></pre>
  
  <p>How can I make <code>(all)</code> say <code>FOO</code></p>