plugin.defaults can be a function, called before anything else, returning the defaults object

this allows the default options to be build dynamically

also allows passing in seneca, and thus gubu, maintaining version matching

```
function myplugin(options) { ... }
myplugin.defaults = (seneca)=>{
  const { Open } = seneca.valid
  return Open({
    settingA: 1
  })
}
```