Added support for extra user variables in ```timeline_render()``` Twig extension
via 3rd parameter.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
* Allowed extra vars in i18n_timeline_render().
* Updated doc.
Merge pull request #171 from phansys/render_timeline_extra_vars

Added support for extra user variables in timeline_render()