Post cover images

Not ready to merge, but a start, I'm using it in production.

- [x] Scripting
- [x] Styling
- [ ] Trigger? - Tag? ID/Class wrapping image in content? Neither?
- [ ] No-js fallback?
- [ ] -webkit-filter: blur?

**NB:** Current implementation assumes you want a post cover image all of the time, no fallback.