Support for language in frontmatter

[lukavdplas]

Add support for specifying notebook language in the frontmatter.

• You can, of course, add a "language" field in the frontmatter yourself, but it would be nice this could be used as the lang property of the HTML document.
• Add the option to select a language like "English", and have it encoded as an ISO-639 code (e.g. "en") in the frontmatter TOML.