Localization of std::io::Error messages

[kellpossible]

On some systems at least (Linux for example) it's possible to have localized std::io::Error messages if the appropriate locale has been set using libc, and the user has the messages installed on their system. It doesn't look as though Rust reads the environment variables in order to do this by default. This proposal is that a feature be added to i18n-embed in order to do that.