Air should support executing binary again when executable exit

[tymonx]

Currently it doesn't execute binary again when exit. It does only when file changes. Air should exit with the same status code as exited binary OR rerun binary again with optional configurable delay. Both features should be configurable and may be disabled on default to keep backward compatibility.

Use case:
Microservices or long-running applications can be run as services under Docker Compose with the restart: always option, Docker Swarm or other container orchestration solution. For development purpose Air can be also run in these environments. But some applications/frameworks terminate running application for example because of unreachable message broker or database service instance. And orchestration rerun them again. But with Air it never happens. Because Air never exit or rerun binary when binary exit by itself.

[xiantang]

good feature.
So air needs to check whether the process is alive, if not alive air should restart the process.
I will do this.