Add a "countdown to next beep" display/process

[mixinmax]

I was trying to get a countdown timer to work alongside the beep process, but got stuck and moved to other things.

The basic requirements are:

• The countdown should be in seconds or in xxhxxmxxs format
• It should update every second
• It should count down to the next scheduled beep
• It should pause when the beep is paused
• It should stop and reset when the beep is reset
• It should use the div/span and css already in the page

It also might be easier to implement if the javascript was refactored.