Opt performance enhancement #334

Hi, I am currently using your lib to parse json logs that can be very large. I was wondering if it would be possible to make opts have an early failure. The reason I am asking is while

try{
return getThing(string);
}catch(exception e){
return defaultValue;
}


is super readable, the throw catch exception is very slow. It would speed things up a lot having a simple check in front of the field to return if there is no field defined from the get go.

Thanks for such a great and easy to use lib.
:)