Add support for loading in a CSV/JSON for custom non-github calendar data

[battlesquid]

There are some users that may not have any activity on github or may have their activity elsewhere. To support them, we should add support for loading in data from a CSV or JSON. I think the simplest format would be an array of day objects that have a date at minimum; other properties can be optional. From there it's just a matter of re-packing the data into a format skyline can work with.

[justinfernald]

It would be fun to pipe in data I get from my extension: https://marketplace.visualstudio.com/items?itemName=JustinFernald.productivity-tracker