[WIP] Selectively enable colors

This PR makes sure colors are only enabled when:

1. The output stream is a TTY
2. Color support is enabled when on Windows (`Win32::Console::ANSI` is defined)

This is a potential fix for #31.

To do:

* [ ] Having `$stdout` in a String extension is ugly. Remove.
* [ ] Having helper methods on `Cri` itself is ugly. Maybe `Cri::Platform`?