Update the time stamp mapping

The following features have been added to the <F2> time stamp function:
    * A bullet is now added at the beginning of the line.
    * A space is added after the ending colon.
    * It will now clear the current line before inserting the
      time stamp. This gets around an issue where Vimwiki automatically
      adds a new bullet point below the current line when you hit enter.
      Without this fix lines started to look like "* * <TIME>....".