tomltestgen command

tomltestgen retrieves a given version of the language-agnostic TOML test suite in https://github.com/BurntSushi/toml-test and generates go-toml unit tests.

Within the go-toml package, run `go generate`. Otherwise, use:

go run github.com/pelletier/go-toml/cmd/tomltestgen -o toml_testgen_test.go