Feature request: Timezone

Not sure if this totally makes sense for this project, though in an app which uses the [`DateTime`](https://hexdocs.pm/elixir/1.12/DateTime.html) module and where each user might have a different timezone set, would it make sense to allow Faker to generate random timezones to help running tests?

I guess the timezone would be represented as a string, with the UTC timezone as "Etc/UTC", and another timezone as "Europe/Copenhagen", for example.

Start by reviewing Faker's existing data-generator entry points and tests; the issue does not name a file or test to begin with. Check how Elixir's DateTime represents timezones and determine the expected generated string format, including "Etc/UTC" and "Europe/Copenhagen". Done means the feature can generate valid random timezone strings for test data.

Written by the indexing model from the issue text.