Rethink index.hbs organization

[budziq]

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

Discussion points:

index.hbs is currently coupled to hbs_renderer and book.js. Book substituting the index with their own, takes a large maintenance burden and looses bugfixes and new features.

• Consider splitting index.hbs into core.hbs (all the js includes and core styles) and index.hbs that would be almost empty and easily replaceable.
• possibly make the other parts replaceable (sidebar, top bar , theme chooser)
• consider adding optional edit button like in rust-cookbook