Make storage backend configurable via -storage.url

Same mechanism as for configuring the auth backend.

storage/url.go

@@ -0,0 +1,24 @@
+package storage
+
+import (
+	"fmt"
+	"net/url"
+
+	"github.com/emersion/go-webdav/carddav"
+)
+
+func NewFromURL(storageURL string) (carddav.Backend, error) {
+	u, err := url.Parse(storageURL)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing storage URL: %s", err.Error())
+	}
+
+	switch u.Scheme {
+	case "file":
+		return NewFilesystem(u.Path)
+	case "postgresql":
+		return NewPostgreSQL(), nil
+	default:
+		return nil, fmt.Errorf("no storage provider found for %s:// URL", u.Scheme)
+	}
+}