Add application log level configuration

This config option ties in with changes in fbd6e77 that were tagged in
v5.2.35 of the framework.

.env.example

 APP_ENV=local
 APP_DEBUG=true
+APP_LOG_LEVEL=debug
 APP_KEY=SomeRandomString
 APP_URL=http://localhost
 

config/app.php

@@ -28,6 +28,20 @@ return [
 
     'debug' => env('APP_DEBUG', false),
 
+    /*
+    |--------------------------------------------------------------------------
+    | Application Log Level
+    |--------------------------------------------------------------------------
+    |
+    | By default, Laravel will write all log messages to its log file. You
+    | can specify the minimum log level by setting this value to one of
+    | the logging levels defined in RFC 5424 and Laravel will log all
+    | messages greater than or equal to the configured log level.
+    |
+     */
+
+    'log_level' => env('APP_LOG_LEVEL', 'debug'),
+
     /*
     |--------------------------------------------------------------------------
     | Application URL