Implement timer for timed notes.

Implement timer for timed notes.

This patch implements a relative time for taking timed notes, useful
for example while watching a video, or during a meeting which is also
recorded.  Here are the new commands:

    - `C-c C-x .' ::
      Insert a relative time into the buffer.  The first time
      you use this, the timer will be started.  When called
      with a prefix argument, the timer is reset to 0.

    - `C-c C-x -' ::
      Insert a description list item with the current relative
      time.  With a prefix argument, first reset the timer to 0.

    - `C-c C-x 0' ::
      Reset the timer without inserting anything into the buffer.
      By default, the timer is reset to 0.  When called with a
      `C-u' prefix, reset the timer to specific starting
      offset.  The user is prompted for the offset, with a
      default taken from a timer string at point, if any, So this
      can be used to restart taking notes after a break in the
      process.  When called with a double prefix argument
      `C-c C-u', change all timer strings in the active
      region by a certain amount.  This can be used to fix timer
      strings if the timer was not started at exactly the right
      moment.