Implements #153: Settings translations (#155)

Implementing a new translating system, so we can more easily translate
settings strings. Indeed, these strings are difficult to embed in
JSON format.

We still use JSON files for standard strings, but in addition we have
dedicated YAML files to translate settings strings.

There is a new build-languages.js script that merge all translations in JSON files.

Other modifications:
* Eslint: ignoring documentation and build scripts.
* french settings translations.

languages/ja.json

@@ -15,5 +15,11 @@
   "Open": "開く",
   "Use current theme colors": "現在のテーマカラーを使用する",
   "Generate an iframe to embed the chat in a website": false,
-  "Chat for live stream": false
+  "Chat for live stream": false,
+  "Room name": false,
+  "Room description": false,
+  "Not found": false,
+  "Video": false,
+  "Channel": false,
+  "Last activity": false
 }