Add support for custom userlists, export format, requirements, and more

[andrewchenk]

This incorporates a lot of changes from another fork I made but from what I can remember:

• Add support for importing and exporting to custom userlists #9
• export_trakt.py now includes all relevant ids in csv
• Make installing requirements easier using requirements.txt
• Updated readme to include custom userlist functionality
• Refactored code to avoid repeating same functions in both export_trakt and import_trakt