filter tarsnap verbose output in list-archives

Thanks a lot for writing tarsnapper!

If tarsnapper is run passing ```-v``` to tarsnap, ```get_archives``` will improperly ignore matching archives due to the extra unexpected tarsnap output.

This pull request detects when tarsnap is running verbosely, and filters ```--list-archives``` output so that it works as-expected.

A couple caveats:

1. Passing ```-v``` to tarsnap doesn't actually do anything user-visible in most cases, as tarsnap output isn't normally displayed.  In my personal fork I log tarsnap stderr and stdout at the debug logging level, but that's a pretty disruptive change.

2. This filter does not tolerate running tarsnap with ```-vv``` or higher, but it looks like tarsnapper's ```-o``` argument doesn't tolerate passing multiple "v"s either.  Again, since running verbosely doesn't really affect user-visible behavior, it's probably not a high priority.  I only make this pull request since README.rst specifically provides an example of passing ```-o v``` to tarsnapper, so this prevents a user walking through README.rst from seeing error messages.