Add multi-stage builder (eg `aio.run.builder`)

We currently have 2 types of runner - a basic runner (as currently defined in `aio.run.runner`) and a checker (as in `aio.run.checker`)

The checker is multi-stage in the sense that it has multiple checks, and some will/not run according to cli args

It would be helpful to add a another runner type - `builder` which like `checker` is multi-stage, and responds to cli args to determine which stages to run

Im thinking this would be useful in particular for integrating the current tools for building docs