Feature Request: ability to add prepend/append code

I.e for running C or C++ or D code i'd have to prepend and append some code before running it

For C it'd be something like
```c
#include

int main() {
// SELECTED CODE GOES HERE
return 0;
}
```

For C++ it'd be something like
```cpp
#include

int main() {
// SELECTED CODE GOES HERE
return 0;
}
```

For D it'd be something like
```d
/+ dub.sdl:
name "test"
+/

import std;

void main() {
// SELECTED CODE GOES HERE
}
```

So possibly allow in file_types to have definition of something like file_template, i.e
```lua
opts = {
file_types = {
d = {
cmd = "dub run --single",
file_template = '/+ dub.sdl:\nname "test"\n+/\nimport std;\nvoid main() {\n${CODE}\n}\n'
}
}
}
```