feat: add template variables

[calebdw]

Hello!

This PR adds some more variables that can be used in the footer template (basically everything available in the slide metadata):

• {title}
• {sub_title}
• {location}
• {event}
• {date}

There may be a better way of including all the metadata in the FooterContext without having to repeat all the definitions?

Default Template

I tried to add a default footer template, but I couldn't seem to decouple the footer template itself from the template style.

The following template is very common in presentations and I think it would make a great default:

left: "{author}"
center: "{title}" # this can also be a short title
right: "{current_slide} / {total_slides}"

Ideally, I would like to just be able to set the style to template and have the default template used:

theme:
  override:
    footer:
      style: template

However, it seems I must manually define the template for all of my slides

Thanks!

[mfontanini]

Thanks!