Add support for fine-grained tokens

[dceuinton]

GitHub are allowing fine grained tokens and it seems like tokens with read-only access to the repositories that need to be backed up would be amazing for this project. However, I couldn't get them to work and had to use the "Classic" tokens.

[mprofitlich]

Thank you for your code addition. I am using your fork for backing with the -all option.

I propose to add a check of all needed rights at the beginning before starting the copy process. Otherwise, if a needed right is not given, errors occur only when the particular copy job starts (in my case copying hooks).