Add a --reset-baseline flag

Currently, `lockcheck` saves the initial scan of dependencies to a `.lockcheck-snapshot.json` file to use as a baseline for future diffs. If developers want to reset this baseline (say, after a massive version upgrade), they have to manually delete the file using `rm .lockcheck-snapshot.json`.

We should add a quick CLI flag to do this automatically.

**Acceptance Criteria:**
-
- Add `--reset-baseline` as an allowed argument in `bin/lockcheck.js`.
- If the flag is passed, check if `resolve(dir, '.lockcheck-snapshot.json')` exists using `fs.existsSync`.
- If it exists, delete it using `fs.unlinkSync()`.
- Print a beautiful green success message `Baseline snapshot deleted. The next run will create a new baseline.` and immediately `process.exit(0)`.