Add `book.translations` to support translation preprocessors

[jooyunghan]

Related to #5

When set, build runs a renderer for each translations after setting book.language to a subdirectory.

For example, when it's set ["ko", "ja"], mdbook build -d book will generate the output:

book/
  (default output)
  ko/
    (ko output)
  ja/
    (ja output)

This enables translation to be handled by preprocessors. For example, it could be PO-file based translation based on book.language.