nest tags because i wanna add mastered tags later

testdata/tofugu.yaml

@@ -8,6 +8,7 @@ knowledge:
 users:
   breel:
     tags:
+      assignments:
       - hiragana
       - katakana
     history: {}

yamldb.go

@@ -19,9 +19,12 @@ type (
 		Answers   map[string]Answer
 	}
 	user struct {
-		Tags    []string
+		Tags    tags
 		History map[string][]History
 	}
+	tags struct {
+		Assignments []string
+	}
 )
 
 func newYamlDB(p string) (yamlDB, error) {
@@ -46,7 +49,7 @@ func (db yamlDB) HistoryOf(user string) map[string][]History {
 	for k, v := range db.Users[user].History {
 		result[k] = append([]History{}, v...)
 	}
-	for _, tag := range db.Users[user].Tags {
+	for _, tag := range db.Users[user].Tags.Assignments {
 		for qid, q := range db.Knowledge.Questions {
 			if _, ok := result[qid]; !ok && q.Tagged(tag) {
 				result[qid] = []History{}