Replaced `colorize` Gem with `pastel`.

Investigating whether mis-formatted output from `help init` was due to
(Windows-focused?) `fazibear/colorize` Gem or to Thor, replaced the Gem
with `peter-murach/pastel`, which incidentally has a few nice features
(currently in the `master` branch, but with an "imminent" formal
release), as embodied by the new `ColourScheme` class. Also, `pastel`
doesn't monkey-patch the String class as `colorize` does.