text: Title: "Minor" longitude and latitude gridlines
  Body: Is there any straight forward way to stride through what tick labels are shown in cartopy? Or could this be added as a feature?
  
  This would be the equivalent of having `xticks=[0,5,10,15,20]` + `xticklabels=['0', '', '10', '', '20']` for instance. The idea here is especially in longitude when labels get packed and overlapped at a reasonable font size for folks to see.
  
  ```python
  import numpy as np
  import proplot as plot
  
  plot.rc['geogrid.linestyle'] = ':'
  plot.rc['geogrid.linewidth'] = 2
  plot.rc['geogrid.color'] = '#d3d3d3'
  
  data = np.random.rand(90, 180)
  lon = np.linspace(-179.5, 179.5, 180)
  lat = np.linspace(-89.5, 89.5, 90)
  
  f, ax = plot.subplots(width='12cm', aspect=4, proj='cyl', tight=True,)
  
  p = ax.pcolormesh(lon, lat, data, )
  
  ax.format(latlim=(20,50), lonlim=(-135, -105), land=True,
            latlines=plot.arange(20,50,5), lonlines=plot.arange(-135,-105, 5),
            labels=True)
  
  ax.colorbar(p, loc='r')
  ```
  
  <img width="364" alt="Screen Shot 2019-08-28 at 2 06 45 PM" src="https://user-images.githubusercontent.com/8881170/63892769-3f444d00-c99d-11e9-856f-f3e9a8cfcf70.png">
  
  This case doesn't have any issues, but imagine if you wanted the label size to be 14, 16, or 18 point font for a poster. All the lon labels overlap. **Any way to show just 130W, 120W, 110W but maintain the grid structure?** I figure there's a means to do it with the formatter/ticker, but I can't figure it out..