Add a web app manifest

_includes/icons.html

@@ -2,3 +2,7 @@
 
         <!-- Favicon -->
         <link rel="icon" type="image/x-icon" href="/favicon.ico">
+        <link rel="apple-touch-icon" sizes="180x180" href="/images/icons/apple-touch-icon.png">
+        <link rel="icon" type="image/png" sizes="32x32" href="/images/icons/favicon-32x32.png">
+        <link rel="icon" type="image/png" sizes="16x16" href="/images/icons/favicon-16x16.png">
+        <link rel="manifest" href="/images/icons/site.webmanifest">

images/icons/site.webmanifest

@@ -0,0 +1,13 @@
+{
+  "name":"mruby Lightweight Ruby",
+  "short_name":"mruby",
+  "icons":[
+    {"src":"/images/icons/android-chrome-192x192.png","sizes":"192x192","type":"image/png"},
+    {"src":"/images/icons/android-chrome-512x512.png","sizes":"512x512","type":"image/png"}],
+  "theme_color":"#e0115f",
+  "background_color":"#ffffff",
+  "display":"standalone",
+  "start_url": "/?source=pwa",
+  "scope": "/",
+  "orientation":"portrait"
+}