Add timer option to configuration

[zubin]

Presenters need to be aware of time.

I'd like the ability to see a MM:SS timer somewhere in the footer (eg greyed out so it's less prominent), eg (see last line):

---
theme: ./path/to/theme.json
author: Gopher
date: MMMM dd, YYYY
paging: Slide %d / %d
timer: true
---

Alternative is to remember to start a timer separately, which is easy to forget. Also when doing a group presentation other presenters might forget.

Additional context
Since the first slide is usually a title, perhaps the timer should start when the second slide is opened. Alternatively, key bindings to start/stop/reset might be simpler and more flexible.

[maaslalani]

This is a great idea, thank you for the issue! ❤️