Configurable countdown!

A huge thank you to @jacquealvesb, who implemented preferences to configure and save a countdown in #5

Screenshot

Changelog (since v0.1)

• Added toggle for 'Show Seconds'
• Migrated to Swift 2.0
• Drastically improve CPU/Energy usage
• Added zero-padding/leading zeros option
• Removed years from countdown
• Swift 4.2 compatibility (#4)
• Fixed bugs with negative times
• Countdown preferences (#5)