Support TOML dates

[mattwidmann]

It would be nice if lua-toml could parse TOML with date primitives in it. The dates could produce Lua date tables (https://www.lua.org/pil/22.1.html) or a custom object with a __tostring meta-method that allows it to round-trip between Lua and TOML.

Ideally, this would support the full set of 0.5.0 date variants: local date-time, local date, local time, and optional T separator.