Clobbering files on extraction and skipping already extracted files

[delcypher]

PRs #34 and #30 both try to make unrarall avoid extracting already extracted files.

Issue #31 reported problems where unrarall would clobber existing rar files upon extraction.

These problems are not independent of each and so they should be tackled together.

Ideally we

• Should not clobber existing files that come from existing archives. E.g. If a.rar contains foo and b.rar contains a different foo and we use the --output flag we'll only end up with one foo file as one will be written over the other. We should avoid this.

• We should handle rar files within rar files (i.e. do recursive extract up to some bound) and should not clobber existing rar files when we do the extraction.

• We should have an option to avoid extracting already extracted files.

The solution to these problems would probably involve examining the archive contents before extracting, always doing extraction in a temporary folder and have a sensible scheme for handling clobbering that emits a warning and tries to carry on by writing the conflicting files somewhere else.

I don't have time to work on this but if anyone wants to tackle this task that would be great.

[arfoll]

Done & merged