add stackqna, xsprogram, stackoom

[tomjnixon]

I'm not sure if you want these as they might just be clones:

*://stackqna.com/*
https://stackoverflow.com/questions/17537167/stddeclval-firing-assertion-error-with-warnings-in-gcc
https://stackqna.com/questions/17537167/stddeclval-firing-assertion-error-with-warnings-in-gcc

*://www.xsprogram.com/*
https://stackoverflow.com/questions/20139415/problems-with-scanf
https://www.xsprogram.com/content/problems-with-scanf.html

*://stackoom.com/*
https://stackoverflow.com/questions/35231982/detecting-instance-method-constexpr-with-sfinae
https://stackoom.com/en/question/2NpRm

Thanks for the great list!

[arosh]

@tomjnixon Thanks. Resolved by #109.