Added DeleteCategory() to Mindflow interface.

pkg/mindflow/mindflow.go

@@ -43,6 +43,7 @@ type Mindflow interface {
 	Categories() ([]Category, error)
 	NewCategory(name string) (int64, error)
 	GetCategoryByID(id int64) (string, error)
+	DeleteCategory(id int64) error
 	DeleteUnusedCategories() error
 	Close() error
 }