Add --cleanup

[lou1306]

Proposal

Add a --cleanup argument to SLiVER's CLI. When sliver.py spec.labs --cleanup is invoked, no analysis is performed. Instead, SLiVER will remove any temporary files that may have been left over from a past analysis of spec.labs.

Rationale

One can already ask SLiVER to --keep-files related to the analysis. This is useful to replicate experiments, debug potential issues with the backend, etc. However, there is no automated way to remove those files once they are no longer needed. This lack of automation could be addressed by adding --cleanup.