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

README.md

@@ -72,7 +72,7 @@ FLAGS:
 
 OPTIONS:
     -b, --base <base>                Base of the input values
--c, --config <config>            Path to the format config
+    -c, --config <config>            Path to the format config [env: FORMATY_CONFIG=]
     -i, --input_type <input-type>    Input data type [default: array]
 
 ARGS:

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"
     )]