Added color classes to flutter icons and its variations to json file

devicon-colors.css

@@ -334,6 +334,10 @@
 .devicon-firefox-plain.colored {
 	color: #DD732A;
 }
+.devicon-flutter-original.colored,
+.devicon-flutter-plain.colored {
+	color:#3FB6D3;
+}
 .devicon-foundation-plain-wordmark.colored,
 .devicon-foundation-plain.colored {
 	color: #008cba;

devicon.json

@@ -343,6 +343,14 @@
       "font": ["plain", "plain-wordmark"]
     }
   },
+  {
+    "name": "flutter",
+    "tags": ["framework", "sdk"],
+    "versions": {
+      "svg": ["original", "plain"],
+      "font": ["plain"]
+    }
+  },
   {
     "name": "foundation",
     "tags": ["framework", "css"],