Support TOML serialization

[magneticflux-]

I have a legacy config in TOML that I need to load using Fiber and I don't want to shadow a whole TOML serde just for a single mod. I saw some discussion about more serializers in #34, but Configurate doesn't support TOML.

I'm ready work on a PR for this change.

[zml2008]

Correctly supporting the toml format and being fully spec-compliant is fairly tricky, and there are good few representation decisions that have to be made to map an arbitrary tree to TOML output. That's why Configurate has not gotten a TOML loader yet, and one of several reasons why I suspect something like fiber would be reluctant to accept a TOML PR.

You also won't get a functional and spec-compliant toml serializer that's much smaller than any of the existing solutions -- so I'd imagine your time would be better spent by just reusing an existing library.

[magneticflux-]

As of mid-2018 the TOML spec (>= v0.5.0) supports "dotted keys", and it's always supported "tables" (which can be nested). The only thing that can't really be represented is a non-branching root node, e.g. a primitive as the whole config.

I'll play around with an implementation some and wrap it into a PR, and I can always split it into another library if the PR is rejected.

Edit:
Found an old issue about this same thing: mwanji/toml4j#17
I also found that there aren't any good TOML serde libraries. TomlJ looks nice and it modern, but doesn't support serialization: tomlj/tomlj#16

I'll probably just convert my simple TOML to JSON and use Jankson going forward.