Allow configuration of time formats

[booniepepper]

Mentioned in https://github.com/hiljusti/sigi/issues/12 - it would be nice if some common time formats could be configured via the CLI

[booniepepper]

Reminder to self:

• https://docs.rs/chrono/0.4.19/chrono/struct.DateTime.html#method.to_rfc2822
• https://docs.rs/chrono/0.4.19/chrono/struct.DateTime.html#method.parse_from_rfc3339

[booniepepper]

Going to say no to this idea. Other tools can do date conversion if necessary