Leverage timer feature to keep track of time spent on active task

[chipsenkbeil]

Use timer feature available in vim 8 and neovim to keep track of progress.

Thought is to have the timer run repeatedly once per second so we can update internal state of time spent on a task.

Would need the ability to have a VimwikiTimeTaskStart (detects task under cursor and starts), VimwikiTimeTaskStop (stops the currently-running task, recording time spent), VimwikiTimeTaskKill (stops the currently-running task, not recording time spent), and VimwikiTimeTaskChange (switches to a new task to log time).

Pomodoro functionality is separate from a timer. A timer just logs the time spent regardless of how long. Pomodoro will be a separate timer of 25 minutes that will stop the current timer for any task and display a floating window to take a break.

[chipsenkbeil]

Store data for this in a comment anywhere in the task. We also need a unique id assigned to it within metadata so we know which task we're tracking given that element forests can be refreshed and lose their node ids.

As the id is just used to be able to find the task when recording, it could be something temporary when active, rather than a truly unique id.