Feature Request: Add option to backup private posts (OAuth authentication)

[magzanilla]

(opening up issue as advised in e-mail)

Similar to the PR for backing up drafts (https://github.com/bbolli/tumblr-utils/pull/200)

Request to please:

1. To add an option to backup private posts on one's own blog
2. add the option to this script's documentation

Thank you very much for maintaining this script despite being busy.