Allow specifying single-character long option names

[eyalroz]

Sometimes you want to automatically specify a bunch of long options using some strings that you're getting - but some of them are one-character-long. cxxopts will treat them as short option names, not long ones. It should at least support a: ",c" syntax for specifying single-characters as long option names.

[SemHeleven]

very sad that this is not implemented. Without this simple option i cannot use your library.