Fix a very nasty conversion bug in Option c'tor

Sometimes C++ can be really bad!

In this case an hard coded c string selects Option c'tor
with int argument instead of the std::string one becuase
it is considered a better matching by the compiler.

Fix the bug changing the argument type from std::string to
const char* so to be a better match then the int one.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>