- Whitelist accepted files to fetch, instead of maintaining an
  incomplete blacklist.
- Clean up after ourselves by deleting files immediately after download.
- Log file moved to `/tmp` instead of project `tmp`, to make sure we get
  cleaned up after.

- When running on accesslint servers, we still want to comment but we
  don't want to try to fetch circle artifacts.

- This allows us to specify an existing logfile that lives locally.
- Moves towards decoupling from CircleCI artifacts.
- Also adds an outfile option to track the diff output between runs.

- Allow a "base" option for specifying a baseline file to compare to.
- Upgrade all gems.
- Split out helper methods from CLI implementation.

- Baseline files can be the initial scan on master or the first scan on
  a PR.
- Subsequent scans will compare results with the baseline to determine
  if there is a diff (more or fewer errors).

- Eliminates warning messages from Thor.

- Up to 10 parallel processes.
- Print command before running for better feedback.

- This makes it easier to run on docker exposed ports that may change
  over time.

- Accept an optional outfile.
- Otherwise record over the previous diff file.
- Fallback to the current directory when there's no previous diff.