Fix flags usage and make Commander Flags capable.

- Programs should exit on Flags parse error.
- Commander now accepts a string vector for interop with Flags.

test/math.cc

@@ -133,6 +133,7 @@ main(int argc, char *argv[])
 	if (parsed != scsl::Flags::ParseStatus::OK) {
 		std::cerr << "Failed to parse flags: "
 			  << scsl::Flags::ParseStatusToString(parsed) << "\n";
+		exit(1);
 	}
 
 	sctest::SimpleSuite suite;