added c++ support

nothing else,

but I got some ideas about a local judge:
1. most of online judges are slow because of simultaneous requests, poor design, racing and limited resources;
2. OJ is default to debug;
3. OJ only tells you the answer is right or wrong;
4. time limits sucks (of course, not for infinite loop);
5. we have extra resources on VPS;
6. show a plot of the running time (analyzing time complexity (automatically or not)), in this way, we can know if there is any better solution;
7. a local environment is helpful to code and debug the code and custom sample test cases; 
8. a quick deployment tool/script helps you install `OJ` on your computer or VPS;
9. ...