add option to include arbitrary tar options

[joequant]

Should add an option to allow user to include arbitrary tar options. This would be useful for adding the ability to do things like incremental backups.

[loomchild]

Hi Joseph, thanks for reaching out, I can take a look into this a bit later. In the meantime, feel free to propose a pull request.

[loomchild]

Done via -x parameter. Thanks for your suggestion and sorry for delay.