#47: Allow custom event-time-format for hikvision

[surdaft]

Fixes #47

Allow custom event time formats for hikvision, I cannot identify why my DVR does not want to return a timezone with the event time, could be because my timezone is UTC? However to support it, I have added the default and ability to support custom formats via the config.

This seemed like a specific camera option and not global, since it can change from one ISAPI implementation to another.

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DVR: Annke N48PBB https://www.annke.com/products/6mp-8ch-poe-nvr