Allow multilevel values for group, platforms and install_if

[paracycle]

So far, Gel has been using only the nearest value defined in a Gemfile for group, platforms and install_if options, disregarding any values supplied in surrounding contexts. However, Bundler actually merges all the values in all the contexts.

This PR is an initial naive attempt to resolve the behaviour difference between the two. It assumes that all such options are Array valued and it just blindly collects all the Array values together.

I would be grateful if I can get comments on the general direction and/or possible improvements on implementation and testing.

[matthewd]

This behaviour seems very odd to me: given a platforms :mri do block, I'd expect any contained platforms: option to apply an intersection, not a union -- it seems weird for the default action to be that it breaks out of the surrounding block's scope. But I think we're obliged to follow Bundler, so it doesn't really matter what I think. 😊

Implementation looks good! Let's not use :rbx, just because I'd prefer to avoid any implication we might support it in the future. (And we'll probably start validating the list some point.) JRuby?

Does this logic apply to source too?