🔧 Create a configuration file interface

This commit adds the ability to provide a config file in different ways:

The following options are given in the order in which they are checked:
1. A config file path passed by the -c parameter on the command line
2. A config file path passed by the HS_CONFIGFILE environment variable
3. A config.json file in the application root

Should these fail we try to get the parameters for the config struct
directly from environment variables or secret files as used in a
container environment. As example the JwtKey can be supplied by either
passing the value through:

* An environment variable HS_CONFIG_JWTKEY which contains the value directly
* An environment variable HS_CONFIG_JWTKEY_FILE which contains a path
  pointing to the Secret File

main.go

@@ -1,8 +1,11 @@
 package main
 
 import (
+	"log"
+
 	"git.beisel.it/florian/hostname-service/api"
 	"git.beisel.it/florian/hostname-service/auth"
+	"git.beisel.it/florian/hostname-service/config"
 	"git.beisel.it/florian/hostname-service/db"
 	"git.beisel.it/florian/hostname-service/docs"
 	"git.beisel.it/florian/hostname-service/middleware"
@@ -29,9 +32,15 @@ import (
 //	@description				Type "Bearer" followed by a space and JWT token.
 
 func main() {
+
+	err := config.LoadConfig()
+	if err != nil {
+		log.Fatalf("Failed to load configuration: %v", err)
+	}
+
 	gin.SetMode(gin.DebugMode)
 
-	db.Init("hostname-service.db")
+	db.Init(config.GlobalConfig.DatabaseFile)
 
 	router := gin.Default()