Make uniform add_param / add_params

[richoux]

add_param takes a std::string as an input, and add_params a std::initializer_list<char const* const>.

However it is easy to forget the final 's' or add an extra 's', and not realizing where the error comes from even after carefully reading the compilation error message. A missing/extra 's' is something easy to skip while reading the error message.

Why not having add_param and add_params taking both kind of parameters, or removing one function? The latter is certainly clearner but will brake some projects using the deleted function.

[adishavit]

I see what you mean.
I don't want to break existing code and I think the current API - with both functions - allows for better readability.

Why don't we just add a std::string overload to add_params() and make add_param() just fwd the call to add_params?

Please make a PR.