Add support for commit and branch snapshots with warnings

Needs docs, but 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.