Add Firebase icon

Added the firebase icon based on the provided source URL, as requested
by @tomap. I choose for the color #ffffff because of "Do not use the
flat knockout in any color but white" listed in the Common Errors
section.

_data/simple-icons.json

@@ -470,6 +470,11 @@
             "hex": "6CC655",
             "source": "https://worldvectorlogo.com/logo/feedly"
         },
+        {
+            "title": "Firebase",
+            "hex": "FFFFFF",
+            "source": "https://firebase.google.com/brand-guidelines/"
+        },
         {
             "title": "Fitbit",
             "hex": "00B0B9",

icons/firebase.svg

@@ -0,0 +1 @@
+<svg aria-labelledby="simpleicons-firebase-icon" role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title id="simpleicons-firebase-icon">Firebase icon</title><path d="M3.89 15.673L6.255.461A.542.542 0 0 1 7.27.289L9.813 5.06 3.89 15.673zm16.795 3.691L18.433 5.365a.543.543 0 0 0-.918-.295l-14.2 14.294 7.857 4.428a1.62 1.62 0 0 0 1.587 0l7.926-4.428zM14.3 7.148l-1.82-3.482a.542.542 0 0 0-.96 0L3.53 17.984 14.3 7.148z"/></svg>