Feature: Add date filter flags

[ryeguard]

This PR implements a new feature. The flags start_date and end_date are simply passed on to the query sent to Garmin when fetching the activity list. This feature is useful both when fetching all historical data for the first time (I sometimes find dates more intuitive than activity count) and when fetching incrementally (the user only needs to keep track of when the script was last run).

I've tested it with my data/account and it seems to work as expected, also in combination with the count and start_activity_no flags which have related functionality.