Added support for extra user variables in timeline_render()

Added support for extra user variables in ```timeline_render()``` Twig extension via 3rd argument.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

TODO: 
- [x] Update documentation