Add support for commit and branch snapshots with warnings

<del>Needs docs, but </del>wanted to get the code up for review. Successfully able to produce my desired commit and branch snapshots using the commands:

```
bikeshed spec index.bs --md-status=Commit --md-warning="Commit deadb33f http://example.com/url/to/deadb33f replaced by https://streams.spec.whatwg.org/" --md-title="Streams Standard (Commit Snapshot deadbeef)"

bikeshed spec index.bs --md-status=Branch --md-warning="Branch ready-fulfill-only http://example.com/url/to/ready-fulfill-only replaced by https://streams.spec.whatwg.org/" --md-title="Streams Standard (Branch Snapshot ready-fulfill-only)"
```

which will be pretty easy to script to run automatically in CI.
Updated