Make the timezone configurable

[billdirks]

When one runs mobility-metrics one passes in a date without a timezone. There is an implicit assumption that the timezone of the machine running the code is the same as the timezone of the city whose data one is querying. That is the start and stop times are derived in cache.js from the variable var day = moment(dayString, "YYYY-MM-DD"); which uses the local system time.

It would be great to be able to configure the timezone, perhaps as a top level key/value pair in the configuration file.