Initialize with authtoken

[rppowellemail]

This change is to make initializing ZabbixAPI with a pre-authenticated token (auth).

This solution is useful for me in a situation where the user/password authentication was handled in a separate script/process as part of daily report generation.

This change works with Zabbix 4.0.0 in a non-production/staging environment.

[coveralls]

Pull Request Test Coverage Report for Build 344

• 2 of 3 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 89.623%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyzabbix/api.py	2	3	66.67%

Totals
Change from base Build 331:	-0.3%
Covered Lines:	285
Relevant Lines:	318

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💛 - Coveralls