Added an after callback function in options

**Why:** I need to execute custom code after all the files have been processed, and need access to what's been created. (Reason is for RequireJS cache busting purposes).

**What's new:** The `after` function is executed after all files are processed, and is passed a list of all the files (same as `callback` option).

Also added tests for the `callback` option, and updated the README.md file.

I hope that makes sense. Please make suggestions in the PR if I should change anything.