Referencing rust-lang#5 (comment).

The book paths have to gracefully degrade to the default language if
they aren't available.

- Add a [language] table to book.toml. Each key in the table defines a
new language with `name` and `default` properties.
- Changes the directory structure of localized books. If the [language]
table exists, mdBook will now assume the src/ directory contains
subdirectories named after the keys in [language]. The behavior is
backwards-compatible if you don't specify [language].
- Specify which language of book to build using the -l/--language
argument to `mdbook build` and similar, or omit to use the default
language.
- Specify the default language by setting the `default` property to
`true` in an entry in [language]. Exactly one language must have `default`
set to `true` if the [language] table is defined.
- Each language has its own SUMMARY.md. It can include links to files
not in other translations. If a link in SUMMARY.md refers to a
nonexistent file that is specified in the default language, the renderer
will gracefully degrade the link to the default language's page. If it
still doesn't exist, the config's `create_missing` option will be
respected instead.

Changes how the `book` module loads books. Now it is possible to load
all of the translations of a book and put them into a single output
folder. If a book is generated this way, a menu will be created in the
handlebars renderer for switching between languages.

We can't redirect in warp based on the URL, so redirect to the default
language's 404 page instead.

See: seanmonstar/warp#171

It will follow relative links to other pages and embedded images.

# Conflicts:
#	.gitignore
#	guide/src/en/cli/completions.md
#	guide/src/en/format/images/rust-logo-blk.svg
#	guide/src/en/format/markdown.md
#	guide/src/en/misc/introduction.md
#	src/renderer/html_handlebars/hbs_renderer.rs
#	src/utils/mod.rs