Add optional setting to remove watched items from watchlists

[RileyXX]

New Feature:

• Add support for optional setting to remove watched items. https://github.com/RileyXX/TMDB-Trakt-Syncer/issues/15

Other Changes:

• Modify opening credentials file to use utf-8 encoding.
• Clean up verifyCredentials.py by modifying the code so all variables are returned from functions.
• Modify all instances of the 'ID' variable to use 'TMDB_ID' instead to make the code easier to understand.
• Modify make_trakt_request() in errorHandling.py to support the params attribute.
• Modify errorHandling.py to import verifyCredentials.py as VC for cleaner coding.