Add group array support to parse_stdin.
Add group array support to parse_stdin.

This gets us back up to two failing tests with the BurntSushi/toml-test
test suite, relating to unicode and escape characters.

vadz/cpptoml,skystrife/cpptoml,Payshares/cpptoml,vadz/cpptoml,Nephatrine/cpptoml,zzzworm/cpptoml,Payshare/cpptoml,Nephatrine/cpptoml,skystrife/cpptoml