Add a simple way to use colours, for individual keys

[rubyFeedback]

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.