feat: ability to add message body from a markdown file

[myint-oo]

❓laravel markdown mails are a lot cleaner than regular blade based mail and the existing view method cannot be used for the markdown files

This PR ✅ adds markdown method just like existing view method which basically does the same thing as laravel mailable markdown method ✅ adds unit test code for the markdown method ✅ updates readme for the new markdown method