* Add search with elasticlunr.js
This commit adds search functionality to mdBook, based on work done by @phaiax. The in-browser search code uses elasticlunr.js to execute the search, using an index generated at book build time by elasticlunr-rs.
* Add generator comment
Someone on Reddit was wondering how the rust book was generated and said they checked the source. Thought I'd put this here. Might be a good idea to have a little footer "made with mdBook", but this'll do for now.
* Remove search/editor file override behavior
* Use for loop for book iterator
* Improve HTML regex
* Fix search CORS in file URIs
* Use ammonia to sanitize HTML
* Filter html5ever log messages
* Created regression tests for the table of contents
* Refactoring to make the test more readable
* Fixed some bitrot and removed the (now redundant) tests/helper module
* Removed the include_str!() stuff and use just the dummy book for testing
* Regression tests now pass again!
* Pinned a `*` dependency to use a particular version
* Made sure test mocks return errors instead of panicking
* Addressed the rest of @budziq's review
* Replaced a file open/read with file_to_string
