title: Using italic() with a variable in ggplot2 title expression
body: <p>When I make a map using <code>ggplot2</code> and attempt to italicize part of the figure title using a combination of <code>expression()</code> and <code>italic()</code> using a string, my map comes out as desired:    </p>
  
  <pre><code>plottitle &lt;- expression(paste('Example map with ', italic('italics')))
  
  crimes &lt;- data.frame(state = tolower(rownames(USArrests)), USArrests)
  states_map &lt;- map_data("state")
  map &lt;- ggplot(crimes, aes(map_id = state)) + 
    geom_map(aes(fill = Murder), 
             map = states_map) + 
    expand_limits(x = states_map$long, 
                  y = states_map$lat) +
    labs(title = plottitle)
  
  map
  </code></pre>
  
  <p>However, when I try to do the same thing, but use an object instead of a string, the object does not evaluate to the desired string:</p>
  
  <pre><code>word &lt;- 'italics'
  plottitle2 &lt;- expression(paste('Example map with ', italic(word)))
  
  map &lt;- ggplot(crimes, aes(map_id = state)) + 
    geom_map(aes(fill = Murder), 
             map = states_map) + 
    expand_limits(x = states_map$long, 
                  y = states_map$lat) +
    labs(title = plottitle2)
  
  map
  </code></pre>
  
  <p>How can I get the vector <code>word</code> to evaluate before applying <code>italic()</code>?</p>