feat: Add flag to set the timezone on reports

[aVolpe]

trafficstars

By default all the dates are in UTC, this commit adds a new flag: '--time-zone' or '-z' that allows the user to pass a timezone.

This is a breaking change, now, by default the used timezone is 'local', meaning the value returned by 'time.local'.

Examples:

• Madrid Timezone: -z Europe/Madrid
• Current timezone: -z local

A full list of timezones can be found here: https://en.wikipedia.org/wiki/List_of_tz_database_time_zones

This can fix some issues reported in #135.

I'm open to fix this PR with your suggestions.

(note that using the time.local timezone is the currently the only option in some report outputs)