Support custom formats (for parsing as well as toString operations)

[fkrauthan]

I think one of the biggest missing items is a formatting system to parse custom date formats as well as to serialize a date object back to a string with custom format. When working with dates both directions are very important and common.

[tadfisher]

A replacement (or delegate) for the ICU RelativeDateTimeFormatter would be a useful addition. The use case is to format localized absolute times, e.g. "today", "yesterday", and "last week".

[tylerwilson]

Hope this is being worked on. I would love to use this package cross-platform, but not having a generic formatting/parsing system is holding us back. Thanks.

[Artrosis]

I needed formatting for logs