Add a simple way to use colours, for individual keys

pp is great.

Sometimes you have a large datastructure, a hash, and it is hard to see which entries belong where to.

For instance I had:

   :files=>
     "/home/x/data/personal_2_bis_6.pdf",
     "/home/x/data/personal2010_Scan.pdf",
     "/home/x/data/personalfoo_8.pdf"],
   :directories=>
    ["/home/x/data/personal/one/",
     "/home/x/data/personal/two",
         "/home/x/data/personal/three",

The real data was much longer.

I'd like to see the keys :files and :directories in another colour, such as blue and green.

These colours should a) have a default, and b) can be customized by the user.

This is for terminal display.

And, the API for this should be simple.

Such as:

pp hash, :default_colours

Or:

Pp.use_default_colours # and then pp

And it should be documented somewhere, ideally here on github too with usage example code.

Start by reviewing the existing pp API and terminal display behavior. Clarify how per-key colours, default colours, and user customization should interact, including the proposed pp hash and Pp.use_default_colours examples. Document the decided API with usage examples and verify it works for large hashes.