text: Title: Make plotting functions work with array-like inputs
  Body: Plots such as `plot_roc_curve` must be able to take any array-like objects. As of today, they only take numpy arrays as input, otherwise an exception is raised. This numpy array conversion must be done inside the function itself.
  
  Example:
  ```python
  skplt.plot_roc_curve([0, 1], [[0.2, 0.8], [0.8, 0.2]])
  ```
  does not work while
  
  ```python
  skplt.plot_roc_curve(np.array([0, 1]), np.array([[0.2, 0.8], [0.8, 0.2]]))
  ```
  does
  