Exclude archive file extensions

[joe9663]

addresses issue #234.

there's probably a different way to exclude these file types automatically, but using this construction has the benefit of giving users the ability to change the configuration if they decide that they want to include archive files in their backups.

[jessecowens]

@bwmarkle Joseph's pull request

[bwmarkle]

@joe9663 Thanks for the PR, and sorry for the wait.

• [ ] Please update the docblock here and tell me what the file extensions represent and why we're adding them. We can look at the code and figure out "what", but not always "why".
• [ ] Can you touch base on how this was tested? I think you've got the right solution, but I just want to be better safe than sorry. What type of scenarios have you thought through? For example, If I've been using TU for a year and then I upgrade, what happens? IE do these default extensions override anything I've previously set? If I haven't changed my settings, will these new settings take effect?