Replace key/common metadata from the challenger, add a config option for which metadata is updated

This is a very similar issue as https://github.com/adobe/aem-experimentation/issues/15 but running into an issue with a customer trying to use this where the `Template` metadata is used to load dynamic js and their challengers are using different template values.

I think we could safely:
* add a config which is an array of metadata fields to update
* default that value to be `['template', 'theme']`
* update/overwrite those metadata fields when overwriting `main`

Start by tracing the code path that overwrites main with a challenger and review the related issue #15 for context. The change is done when an array configuration controls which metadata fields are replaced, defaults to template and theme, and those fields are updated when main is overwritten.

Written by the indexing model from the issue text.