Add optional attribute 'driver' to config (#404)

This will help to use custom mock drivers or replacements.
Full credits to @0nedark whose PR branch was deleted before the review.

connection.go

@@ -94,7 +94,12 @@ func (c *Connection) Open() error {
 		return errors.New("invalid connection instance")
 	}
 	details := c.Dialect.Details()
-	db, err := sqlx.Open(details.Dialect, c.Dialect.URL())
+	driver := details.Dialect
+	if details.Driver != "" {
+		driver = details.Driver
+	}
+
+	db, err := sqlx.Open(driver, c.Dialect.URL())
 	if err != nil {
 		return errors.Wrap(err, "could not open database connection")
 	}

connection_details.go

@@ -16,8 +16,10 @@ import (
 
 // ConnectionDetails stores the data needed to connect to a datasource
 type ConnectionDetails struct {
-	// Example: "postgres" or "sqlite3" or "mysql"
+	// Dialect is the pop dialect to use. Example: "postgres" or "sqlite3" or "mysql"
 	Dialect string
+	// Driver specifies the database driver to use (optional)
+	Driver string
 	// The name of your database. Example: "foo_development"
 	Database string
 	// The host of your database. Example: "127.0.0.1"