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

[shsteimer]

This is a very similar issue as #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