Add backup file adder code

[nrathaus]

Description

Add support such that when a file is found (200 HTTP OK code), additional extensions (related to backup files) are added to the queue

TODO

Some functionality is done quite ugly, let me know how you want to support it

1. backup extensions are hardcoded to a list, should I read it from a file? a config parameter?
2. _items is directly accessed, should I add a Dictionary func to add items? is it allowed to add to _items running it appears to have no ill effect

[NEX-S]

backup extensions are hardcoded to a list, should I read it from a file? a config parameter?
For the sake of extensibility, I think we should provide custom prefixes/suffixes using the parameter