Mostly some small bugfixes, with a few small new features:

• Add Position.Col, to mark the column an error occurred (#410)

• Print more detailed errors in the tomlv CLI.

• Ensure ParseError.Message is always set (#411)

• Allow custom string types as map keys (#414)

• Mark meta keys as decoded when using Unmarshaler interface (#426)

• Fix encoding when nested inline table ends with map (#438)

• Fix encoding of several layers of embedded structs (#430)

• Fix ErrorWithPosition panic when there is no newline in the TOML document (#433)