Add Postgres and MySQL connection strings (#429)

* Add ConnectionURI to Postgres

* Add ConnectionURI to MySQL

* Update Postgres sample db name

postgres/README.md

@@ -49,12 +49,24 @@ store := postgres.New(postgres.Config{
 	GCInterval:      10 * time.Second,
 	SslMode:         "disable",
 })
+
+// Initialize custom config using connection string
+store := postgres.New(postgres.Config{
+	ConnectionURI:   "postgresql://user:password@localhost:5432/fiber"
+	Reset:           false,
+	GCInterval:      10 * time.Second,
+})
 ```
 
 ### Config
 ```go
 // Config defines the config for storage.
 type Config struct {
+	// Connection string to use for DB. Will override all other authentication values if used
+	//
+	// Optional. Default is ""
+	ConnectionURI string
+
 	// Host name where the DB is hosted
 	//
 	// Optional. Default is "127.0.0.1"
@@ -105,6 +117,7 @@ type Config struct {
 ### Default Config
 ```go
 var ConfigDefault = Config{
+	ConnectionURI:   "",
 	Host:            "127.0.0.1",
 	Port:            5432,
 	Database:        "fiber",