Add support for Content Tags

[Jonathan-Zollinger]

Support content tags for Articles only

• Search Content Tags
• Count Content Tags
• Show Content Tag
• Create Content Tag
• Update Content Tag
• Delete Content Tag
• Create Content Tags Job

This is complete when docs are supplemented with example usage (both successful calls and failures), and tests validate Zendesk's documented behavior - successful calls and failures (i.e. the updateUserSegment endpoint returns a 200 error code and the userSegmentObject created, thus tests should validate this and nothing more).