Rethink book.js organization

[budziq]

As discussed in https://github.com/azerupi/mdBook/pull/338#discussion_r123618836

Discussion points:

• Consider splitting book.js by feature
  • Separate into smaller js files included in index.hbs?
  • book.js generated like index.hbs?
• Consider switching to TypeScript for type safety.
  • Adds dependency on TypeScript compiler
  • Slightly harder to start contribution for newcommers

[sorin-davidoi]

In #556 book.js was split into modules, but they are still in one file.