Support datetime formats other than unix epoch about weewx-mqttsubscribe

[bellrichm]

I’d really like to implement this without requiring any modules that are not currently used by WeeWX. So, this would eliminate pytz, dateutil, dateparser....

[bellrichm]

I’ll be leveraging datetime.srtptime to create the datetime object. It seems that in python 2 its support of timezones is iffy at best. So, I plan to manage that with a separate format string for the time offset.
So for example:
datetime string=2016-09-20T16:43:45-07:00
datetime format=%Y-%m-%dT%H:%M:%S
offset format=hh:mm

datetime string=2016-09-20T16:43:45
datetime format=%Y-%m-%dT%H:%M:%S
No offset format

datetime string=September 20 2016 16:43:45 -0700
datetime format=%B %d %Y %H:%M:%S
offset format=hhmm

The offset format is either hh:mm or hhmm.