[Feature request] Support compile&link flags in comments

**Is your feature request related to a problem? Please describe.**
When use code runner with c/cpp codes, we need to add various compile/link flags for third-party libraries. Even though we could add all *potentially* needed libraries in `code-runner.executorMap.cpp` as an all-in-one solution, but it's too ugly and not portable across machines.

**Describe the solution you'd like**
Consider we have code like this:
```cpp
// vscode-runner: -std=c++11 -I/opt/fmt/include -L/opt/fmt/lib -lfmt
#include

int main() {
fmt::print("hello world\n");
return 0;
}
```
We could extract compile command flags from special comments `^\/\/\s*vscode-runner:\s*(.*)$`, instead of modify settings.json.
And then the default config of `code-runner.executorMap.cpp` would be `"cpp": "cd $dir && gcc $commentFlags $fileName -o $fileNameWithoutExt && $dir$fileNameWithoutExt"`