Support `std::string_view`

[schaumb]

With the current implementation, the option handling/parsing only works with a lot of allocations. If the implementation would use string_view internally and can save arguments as viewer structs, that could be neat.

[Ahajha]

I'm interested in this as well, I'm starting a branch and seeing how much can be done. I don't know if this project still takes PRs, but if not I'll leave mine up as a fork.

[Ahajha]

I got most of the low hanging fruit, I might make a second pass later to get more.