Handle SIGTERM properly

[denverdino]

Signed-off-by: Li Yi [email protected]

Fix #182

Tested in Mac/Linux

$ cargo run -- -c examples/modules.toml
    Finished dev [unoptimized + debuginfo] target(s) in 1.03s
     Running `target/debug/wagi -c examples/modules.toml`
No log_dir specified, using temporary directory /var/folders/jh/x0hxxwzn7j7fhmlh3q3k17x40000gp/T/.tmp9evE6D for logs
Ready: serving on http://127.0.0.1:3000

In the separated terminal

$ kill -15 PROCESS_ID

[ghost]

All CLA requirements met.

[itowlson]

Sorry for the lack of reply - I've been away. Thanks for this - I'll take a look!