Hello!

I'm dealing with ISO8601 strings for date times, and the current libraries 
make my use case, which is dealing with clock time but preserving the 
offset from UTC, very frustrating.

Specifically (unless I'm mistaken) there is no inbuilt functions that allow 
reversible parsing of them.

e.g.

Given "2020-09-7T10:59:00+0100" I can parse this into a UTC DateTime and an 
offset, or a NaiveDateTime ignoring the offset, but from either of these I 
cannot reproduce "2020-09-7T10:59:00+0100".

I understand that an offset is not a timezone, so I understand why by 
default it does get converted to UTC, but it does represent the clock time 
at a point in time (which is useful for historical time series data like 
I'm working with).

What I propose is two options:

1) Have either/both DateTime and/or NaiveDateTime's `to_iso8601/2` expanded 
to take an offset back for the point of formatting, e.g.

```
{:ok, utc_dt, offset} = DateTime.from_iso8601("2020-09-7T10:59:00+0100")
"2020-09-7T10:59:00+01:00" = DateTime.to_iso8601(utc_dt, :extended, offset)
```

2) Have NaiveDateTime capture offset into it's struct, and allow it to be 
included when formatting date times.

```
{:ok, naive_dt} = NaiveDateTime.from_iso8601("2020-09-7T10:59:00+0100")
"2020-09-7T10:59:00+01:00" = NaiveDateTime.to_iso8601(naive_dt, 
:extended_offset)
```

I'd be happy to work on either / both of these, but currently I have to use 
DateTime.add and manually edit the struct to get the result I want, and 
given that the inbuilt libraries are more than sufficient for all my other 
purposes the current iso8601 handling feels like it could be improved

Cheers and look forward to feedback
Jon

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