Add detection for Flat UI (#1976)

The official website is [here](https://designmodo.github.io/Flat-UI/),
the github is [here](https://github.com/designmodo/Flat-UI), and can be
tested [here](https://up2sha.re/).

src/apps.json

@@ -2996,6 +2996,15 @@
       "implies": "Python",
       "website": "http://flask.pocoo.org"
     },
+    "Flat UI": {
+      "cats": [
+        "18"
+      ],
+      "icon": "Flat UI.png",
+      "implies": "Bootstrap",
+			"html": "<link[^>]* href=[^>]+flat-ui(?:\\.min)?\\.css",
+      "website": "https://designmodo.github.io/Flat-UI/"
+    },
     "FlexCMP": {
       "cats": [
         "1"