text: Title: Custom axes titles for learning curve subplots
  Body: Please add options to add custom x, y axes labels when plotting subplots for learning curves.
  At present unable to add them. Also same goes with xticks, yticks.
  
  Xticks: As you can see for my implementation there's no fixed xticks for each subplots. Make an option to add custom xticks.
  
  ![image](https://user-images.githubusercontent.com/23068132/81775905-54e9aa80-950b-11ea-87f0-8eef81084147.png)
  