Add tmux-continuum status plugin

[aaronkollasch]

This PR shows the status of the latest tmux-continuum save and a countdown to the next save.

If no save is present:

When a save occurs:

Then the timer resets:

[xiantang]

well done

[ethancedwards8]

Code looks good. I'm just not sure if this is feature creep or not. Can you justify this instead of putting it in a script?

[aaronkollasch]

It's up to you. I did a brief survey of tmux plugins and tmux-continuum has 2.7k stars, more than tmux-yank, about 1/4 of tmux-resurrect or the plugin manager tpm, and about 1/10 of tmux itself. So I'd say tmux-continuum seems like a popular plugin. Also, if it's a separate script I'm not sure how people would find it.

[ethancedwards8]

That's fair, thanks for the contribution!

[aaronkollasch]

Thanks for reviewing!