add "no-link" cargo feature

[danlarkin]

Using a cargo feature is a bit easier for projects that never want to link to libruby - like those building a ruby extension in rust.

[calavera]

I'd love to see this merged. Even if it's not published in a new release, having it in master means that you could use it by depending on this github repository.

[danielpclark]

Looks Good!