Generate reports on pomodoros and task tags by date range

[qualidafial]

Depends on #7.

Where I work they want CapEx reports after the end of each month to know how much time was spent on certain projects during the previous month, for their financial reporting. Also I just want a way to see how my time is being spent.

Once we have task tagging add a feature that generates a reports the number of pomodoros spent on each tag for a specified date range. Pomodoros with multiple tasks will be divided up evenly between each task. For example, in a pomodoro with two tasks, each task will be assigned 1/2 of a pomo.

Include untagged tasks as their own category.