x-mojo-stash values should be added to the stash

```
/paths:
  /foo:
    get:
      x-mojo-stash:
        foo: 1
        bar: 2
...
```

After successful request validation, copy all values from the x-mojo-stash operation object into Mojo's stash.

Perhaps have a plugin option that warns (or dies?) when overwriting an existing stash value.