Allow adapters to specify custom help text

e.g. when the user types

```
run-tests jest --help
```

The CLI should display some sort of help text from the adapter itself if specified, allowing each adapter to document any specifics of its behaviour.

One possibility to is to have adapters export a `getHelpText` function similar to executeTests.

Start by locating the CLI handling for `run-tests --help` and the adapter interface around `executeTests`. Trace how adapter-specific options are dispatched, then define and document how custom help text is supplied and displayed; done means the Jest example shows adapter-provided help when requested.

Written by the indexing model from the issue text.