Use conan to add dependency on CLI11

(Also use some basic CLI11 to verify build/link works.)

src/cppchallenge.cpp

@@ -1,6 +1,9 @@
-#include<iostream>
+#include "CLI/CLI.hpp"
+#include <iostream>
 
 int main(int argc, char *argv[]){
+   CLI::App app{"Just a simple 'hello'"};
+   CLI11_PARSE(app, argc, argv);
    std::cout << "Hello World!" << std::endl;
    return 0;
 }