Goal, sum, theme and 3.8!

After more than a year a new release brings few improvements:

• Python 3.8 support
• new toggl sum command that displays sums of time grouped by days
• new toggl goal command that waits until you reach your defined goal for the day and the send notification
• new theming support. For already existing configs you can set it in config under the theme option. See documentation for more details.
• --today flag for ls and sum command