Require 2-digit hour for local time

[moorereason]

Given a=1:32:00, this package returns a valid time-local value instead of returning an error.

Reproduction

https://play.golang.org/p/fciY3UVyoT6

toml.Unmarshal([]byte("a=1:32:00"), &v)

Or using the toml-test decoder:

$ echo "a=1:32:00" | toml-test-decoder2
{
  "a": {
    "type": "time-local",
    "value": "01:32:00"
  }
}

[arp242]

This is actually an issue for all datetimes, not just local time; the problem is that 15 in time.Parse accepts both 01 and 1. There doesn't seem any way to require a leading zero.

[arp242]

I'm somewhat leaning towards just leaving it like this; writing times like "1:12:13" isn't too uncommon, it will always encode things as "01:12:13" so it won't cause issues with other decoders, and even though it's not fully spec-compliant I don't really see any problems with it. Might be worth discussing modifying the specification on this.

[moorereason]

I'm simply reporting issues I've found while doing differential fuzzing against this package and go-toml and toml.dart.

But I'm in favor of following the TOML spec. As reported, this issue doesn't follow the spec.