Update commit message standards to reflect conventional-changelog acceptance

[jthomerson]

In lambda-express we have started using conventional-changelog syntax for our commit messages, and enforcing it in our linting. We use commits to bump versions correctly and generate changelogs. If we decide we like that format, we will need to update our coding standards here to document it.

When updating the standards, be sure to explicitly state:

• line lengths
• types / scopes
• indentation (e.g. starting lines after BREAKING CHANGE:\n with four spaces so that they get properly indented in the changelog list item - probably the same for lists that we explicitly write in our commit bodies, too)
• format for referencing multiple issues

[jthomerson]

Make sure to include all the stuff we commented on and codified in this PR:

https://github.com/silvermine/eslint-config-silvermine/pull/44/files