Fix arguments after -- without declared positional

Fixes #36. This fixes handling arguments passed after `--` when no
positional arguments have been declared, or when the positional
arguments have all been used up. The bug was that the extra arguments
were lost. Now they are retained in the modified argv.

test/options.cpp

@@ -153,3 +153,25 @@ TEST_CASE("Some positional explicit", "[positional]")
   CHECK(positional[0] == "c");
   CHECK(positional[1] == "d");
 }
+
+TEST_CASE("No positional with extras", "[positional]")
+{
+  cxxopts::Options options("posargmaster", "shows incorrect handling");
+  options.add_options()
+      ("dummy", "oh no", cxxopts::value<std::string>())
+      ;
+
+  Argv av({"extras", "--", "a", "b", "c", "d"});
+
+  char** argv = av.argv();
+  auto argc = av.argc();
+
+  auto old_argv = argv;
+  auto old_argc = argc;
+
+  options.parse(argc, argv);
+
+  REQUIRE(argc == old_argc - 1);
+  CHECK(argv[0] == std::string("extras"));
+  CHECK(argv[1] == std::string("a"));
+}