feature: Add chapther numbers to search result breadcrumbs

[WofWca]

This would make the search results easier to scan is some cases (e.g. when the book is big (like The Book).

Before:

After:

It could be implemented as an optional thing.

Here's where they're being generated:
https://github.com/rust-lang-nursery/mdBook/blob/77af889a2e5331c11345ddef935caccb0305cd34/src/renderer/html_handlebars/search.rs#L92

May be better to first make some architecture changes that would make it easier to implement
A quote from #631:

The idea is a nested chapter can have links back to its parent (and maybe siblings?), making things like section numbers and breadcrumbs a lot easier to implement.