Add support for highlighting breaking API changes

[jverkoey]

From alexisgallagher:

Does it auto generate an indication when the API change breaks compatibility for code that depended on the old API?

Things we need to do to make this happen:

• [ ] Add a new API to ApiChange for isBreaking().
• [ ] If an ApiChange is a breaking change, output additional markdown highlighting this fact.

It may make sense to aggregate breaking API changes near the top of the output as well.