title: cartopy set_xlabel set_ylabel (not ticklabels)
body: <p>When using a cartopy map I cannot add an xlabel or ylabel. Is there a way to do this? I am not looking for ticklabels.</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import cartopy
  ax = plt.axes(projection=cartopy.crs.PlateCarree())
  ax.add_feature(cartopy.feature.COASTLINE)
  ax.set_xlabel('lon')
  ax.set_ylabel('lat')
  plt.show()
  </code></pre>