Additional format options

Two changes:

- Enable video formats as an option. I have left the default unchanged. Making this change so we can enable flv ads (as seen [here](https://github.com/guardian/frontend/blob/f0f4712731ea4681ad7ddd770ce94a1ad5050364/common/app/assets/javascripts/bootstraps/media.js#L346))
- Format mapping to enable treatment of one mime-type as another. Making this change as some ad networks give bizarre custom mime-types like video/x-mp4 for plain video/mp4.