add ability to view specific date range within Pomodoro history

[finnigantime]

This allows the history tab Pomodoro data to be filtered to a specific date range. This allows the user to, for example, visualize their Pomodoro history for just 2019.

• Partially addresses More advanced statistics #75
• I don't have much experience with Chrome extensions or Vue so not sure I did things correctly. I went off existing patterns I saw in the codebase. Feel free to call out anything weird.
• The timestamp calculation on the beginning/end dates may be slightly off - I wasn't rigorous about zeroing out hh:mm:ss to the start of the day or dealing with timezones. But other than that it should work and most of the time you'd care to view data over a longer multi-day range, and you can always choose a day earlier or later than the range to safely include all data.
• I only added locale strings for English. Is that okay?

before:

after: