Add Tags and tag types

- First iteration of api for tags and tag-types
- Documentation in place
- Adds three new tables
   - tag_types
   - tags
   - feature_tag
- Tagging a feature is adding a row in the feature_tag
  join table

* #665

Co-authored-by: Simen Bekkhus <sbekkhus91@gmail.com>
Co-authored-by: Ivar Conradi Østhus <ivarconr@gmail.com>

lib/command-type.js

@@ -0,0 +1,9 @@
+module.exports = {
+    TAG_FEATURE: 'tag-feature',
+    UNTAG_FEATURE: 'untag-feature',
+    CREATE_TAG: 'create-tag',
+    DELETE_TAG: 'delete-tag',
+    CREATE_TAG_TYPE: 'create-tag-type',
+    DELETE_TAG_TYPE: 'delete-tag-type',
+    UPDATE_TAG_TYPE: 'update-tag-type',
+};