Deal with local datetimes, dates, and times better

[arp242]

The exact behaviour of a local datetime is not precisely defined:

If you omit the offset from an RFC 3339 formatted date-time, it
will represent the given date-time without any relation to an
offset or timezone. It cannot be converted to an instant in time
without additional information. Conversion to an instant, if
required, is implementation-specific.

So this is how this implementation will deal with it:

• Use the local timezone, for example "WITA +0800" in my case.

• However, use the new toml.Local* timezones to do so instead of
  time.Local. This has the same offset, but you can compare/use this:

  if mytime.Location() == toml.LocalTime {
  ...
  }

Without this information it's hard to know if we're dealing with a date,
time, or local datetime. How would I distinguish between a "datetime
with the TZ set to whatever my local time zone is" and "a local
datetime"? Or how do I know that the time 00:00:00 means it's not
specified at all vs. just midnight?

An alternative solution would be add this to the MetaData, but I think
this is nicer as it's part of the time.Time.

The downside is that "fmt.Println(mytime)" will print "local-datetime"
as the "timezone". I think that's okay, and actually useful as it
signals you forgot to do something with this local date. As the spec
says:

It cannot be converted to an instant in time without additional
information.

(Aside: to be honest I'm not sure this entire feature was really a good idea –
times and timezones are already hard enough, and you can represent this
kind of stuff as just strings too; but it's in there so we have to deal
with it.)