Add support for RelativeDateTimeFormatter

[ubuntudroid]

Android supports displaying date and time relative to some timestamp. However, the implementation suffers from the same 12h/24h clock issues like many of the other formatters.

It would be great if this library would also provide a formatter for this scenario and maybe also provide adapted versions of the android.text.format.DateUtils.getRelative* convenience methods.

[drewhamilton]

How does the 12/24h clock setting affect RelativeDateTimeFormatter? "13 hours ago" should be the same regardless of that setting.

DateUtils.getRelative... does require epoch milliseconds, so I could see an argument for providing similar functions accepting LocalDate etc.

[ubuntudroid]

To be honest, I don't recall what I wanted to say with that 12h/24h thingy back then. 😓 There was a specific case I had in mind - I should have explicitly written it down here. I'll do that once I stumble on it again.

LocalDate accepting relative functions would be very much appreciated, but no need to hurry with that - it is simple enough to convert an epoch time to a LocalDate after all.