Idea: more flexible bundler config

[hurricup]

As far as I get it, current bundler config (above project level) is shared between all rubies I might have.
But sometimes, some options are necessary only on particular version of ruby.

E.g. I encountered an issue installing nokogiri 1.6.6.4 on macOs for ruby 2.3 and 2.4. And this may be fixed with bundle config build.nokogiri --use-system-libraries

I'm not sure if it will harm other versions, where nokogiri installs fine without it, but I can easily imagine case when something like this happen.

So, it feels that some bundler options could depend on ruby version in use, and some, like build option, could be more precise, like include gem version constraints.

Or, another ruby-level config could be introduced to solve part of the issue. This feels like cheap solution.

[hurricup]

Or even not ruby version, but ruby path. I may have several rubies of the same version, like system and rvm one and they have their quirks