Add security commits to release notes

[GuySartorelli]

#212 adds functionality to automatically generate release notes and create a github release after tagging the release.

The release notes don't include security commits.

Acceptance Criteria

• When cow creates a github release, the release notes include security commits.
• The security commit information matches the formatting of the other commits in the release notes

Notes

The release notes are generated by github via the github API. Most likely you will need to have cow check if it has any security commits (use similar logic as when creating the changelog) and if so add information about those commits into the release notes.