support multiple BREAKING CHANGE footers

**Is your feature request related to a problem? Please describe.**

Sometimes, a commit may introduce more than a single breaking change, in which case it makes sense to have multiple separate lines for each breaking change in the changelog. This is currently not possible, because `release-please` only uses the last `BREAKING CHANGE:` footer in the commit message.

**Describe the solution you'd like**

I would like that each separate `BREAKING CHANGE:` footer creates a new line in the "BREAKING CHANGES" section of the changelog.

**Describe alternatives you've considered**

We considered writing everything in the same footer, but since it is not possible to add new lines in the changelog entry, that is suboptimal.

**Additional context**

See https://github.com/cheminfo-js/test/pull/11 for an example release PR created from commit https://github.com/cheminfo-js/test/commit/aa9c24e0a229e93845de160e44d276ac6506219c