Allow directly customizing the creative and line item configs

[kmjennison]

People often want to customize the creative and line item configuration. Rather than adding case-by-case customization, let's just expose the configuration for LineItem and Creative (and/or ThirdPartyCreative) to make it more flexible.

This could address the following feature requests:
SafeFrame: #83
Same advertiser exception / geotargeting: #59
Video ads: #61
Native ads: #57

This is straightforward for boolean fields but more complex for any items we need to create before associating them with the config, such as custom targeting keys.

It might be helpful if #50 is a prerequisite to this feature.