Support preexisting hooks in .pre-commit-config.yaml

[tdurk93]

When seCureLI writes a .pre-commit-config.yaml file, it does not recognize or respect any existing contents if that file already exists. We should merge the YAML with existing hooks. We also need to be able to track which hooks were added by seCureLI, so that any changes due to updates (or removal of seCureLI) can avoid making changes to pre-existing hooks.

Open questions:

• How do we track if one or more hooks added by seCureLI are duplicated with the pre-existing functionality?
• If we install different/additional hooks from a repository that is already in use, should we add those new hooks to the existing repository listing? If we instead opt to duplicate the repository, do we have to match the repo version with the existing config?