Add a `--section=<string>` flag that uses ropo to only replace a section of `--write=`

[balupton]

Doing this would also require moving the section rendering code from projectz into the github-api CLI, but that is fine. Doing such will also assist in disassembling projectz into smaller composable utilities with CLIs for using without projectz, with APIs that projectz just consumes.

Relevant projects:

• https://github.com/bevry/ropo
• https://github.com/bevry/projectz
• https://github.com/bevry/argument

Other projects that would benefit from such:

• https://github.com/bevry/badges

And while I'm at it, these projects should have CLIs added now that bevry/argument and bevry/fs-* tooling makes Node.js CLIs really easy:

• https://github.com/bevry/arrange-package-json