Add a flag to clean up cloned repos after execution or not clone at all

[mccurdyc]

Description

• there should be a way to clean up repos after execution

Example

• maybe I want to add a LICENSE file to all repos, and then be done with the repos after that

Thoughts

• a flag (--clean-up or --clone)
• to me --clean-up=true|false suggests that the repos would actually be cloned to disk and then after execution, would be removed
• --clone=true|false to me suggests that someone would never be written to disk and instead would just use memory (e.g., https://godoc.org/gopkg.in/src-d/go-git.v4#Clone)