Support toml schema validator

[bungcip]

vscode have nice feature when opening .json file. The editor will validate the file based on http://schemastore.org schema.

I want better-toml to have that feature. Initial implementation will be based from vscode json extension and using json schema.

If someday toml have support for schema validation file, better-toml will also support it (I still hope for toml-lang/toml#116)