text: Title: [DOCS] Explain matplotlib get_cmap function
  Body: Would it make sense to implement a `get_cmap` function as `colorcet.cm.get_cmap` that could override Matplotlib's so that external libraries using MPL's `get_cmap` could just replace it with Colorcet's `get_cmap` if Colorcet is available at runtime? This would make interfaceing with Colorcet super easy for other libraries already leveraging MPL's colormaps.
  
  I'm thinking that this function would have the same arguments and behavior as MPL's `get_cmap` but it would also grab Colorcet colormaps by name and default to Colorcet if MPL has a similarly named colormap.