Added an option to display a custom footer

Added an option in config file.
Added a component which renders the custom footer or a default footer if
no custom footer is defined.
Updated readme file with related instructions.

package.json

@@ -1,7 +1,7 @@
 {
   "name": "@arifszn/gitprofile",
   "description": "Create an automatic portfolio based on GitHub profile",
-  "version": "2.1.0",
+  "version": "2.1.1",
   "license": "MIT",
   "author": "arifszn",
   "repository": {