Add environment fallback for config path

+ FORMATY_CONFIG is the fallback for the optional config path
+ If -c is not provided, we go to the system config

src/main.rs

@@ -16,6 +16,7 @@ pub struct Args {
     #[structopt(
         short = "c",
         long = "config",
+        env = "FORMATY_CONFIG",
         parse(from_os_str),
         help = "Path to the format config"
     )]