Allow specification of `std=c++17` for tests

[jgfoster]

I see that cpp_library.rb:401 specifies -std=c++0x as the version of C++. I'd like to use some features that are in std=c++17 (I want the filesystem features). Would it be reasonable to bump up to version 17 or make it optional?

[ianfixes]

I'm actually falling somewhere in between, see #146

Summary: I think this project should be using the same hard-coded c++ standard as the official Arduino compiler(s). It's not doing that yet.

So to be blunt, in my opinion this is either a duplicate of #146 or a won't-fix.

[jgfoster]

We found it pretty difficult to mock SD in a cross-platform way without having filesystem. So, I'll make a gentle appeal for at least an option to use a later library at your own risk. But I'm sure we can find a work-around if needed...

[jgfoster]

I've had further thoughts on this and will present them in a new issue.