Decoder: support all three kinds of TOML dates

[jmelfi]

Currently datetime is having an issue with being able to be rendered as in the v0.4.0 spec. If you use a time that does not use UTC (e.g. doesn't have a 'Z' at the end) you get a failure.

1979-05-27T07:32:00Z will convert properly but 1979-05-27T07:32:00-08:00 will not.

I hazard a guess that this ties to the parse.go mapping Datetime

    case itemDatetime:
        t, err := time.Parse("2006-01-02T15:04:05Z", it.val)
        if err != nil {
            p.bug("Expected Zulu formatted DateTime, but got '%s'.", it.val)
        }
        return t, p.typeOfPrimitive(it)

Is this change in the works or does this need to be started on? I'd be glad to help get this worked in. The encode.go appears to need a change as well to have encoding be able to use UTC or non-UTC based time.

func (enc *Encoder) eElement(rv reflect.Value) {
    switch v := rv.Interface().(type) {
    case time.Time:
        // Special case time.Time as a primitive. Has to come before
        // TextMarshaler below because time.Time implements
        // encoding.TextMarshaler, but we need to always use UTC.
        enc.wf(v.In(time.FixedZone("UTC", 0)).Format("2006-01-02T15:04:05Z"))
        return

If you have any recommendations on this, I may have time to work a bit on this later this weekend. This lack of support blocks movement forward on another project that relies on this library.

[cespare]

Since the issue description is about TOML 0.4.0 dates specifically, I have changed the issue title. (I'll make a meta-issue or milestone for 0.4.0 later, and take stock of all the missing features.)