Improve 'io-console' loading

__tl;dr party_foul 1.1.1 is broken on 1.9.2__

Since the gemspec is evaluated at **build time** and not runtime like Gemfile, ad0435f08ed72030fa58353eb3d154224960747c means that whether the gem depends on `io-console` is determined by which platform you build it on.

This is a common problem when trying to specify different dependencies for different platforms, and the gemspec doesn't really support it.

IMO, it'd be nice to get rid of the io-console dependency entirely, but failing that, this PR uses a similar approach to gems with the same problem, e.g. [comma][comma], [zeus][zeus]


[comma]: https://github.com/crafterm/comma/blob/78bceb282ff1234a9ca31fc586f8c75edf3cf34b/lib/comma.rb#L2-L14

[zeus]: https://github.com/burke/zeus/commit/1dd4e81