Load Windows color support properly

This is a potential fix for #352. It makes sure `'Win32/Console/ANSI'` is required as soon as possible, i.e. when the `cli.rb` file is loaded. (This should have been here all along, because it does not really make sense to load color support when setting up cleaning streams.)

Minor changes:

* Created `Nanoc.on_windows?` method that is referenced everywhere else, so that the `RUBY_PLATFORM` check only happens in a single location

* Added many more Windows-ish platforms to the `RUBY_PLATFORM` check (not strictly necessary, but better to do it right, I figured)

@raphinesse Can you verify whether this patch works on Windows?