Add rewrite_to_dir config to rewrite index file path

[weihanglo]

The PR is for

Rewrite specified filename of URLs to directory index /.

Intent

Need cleans URL for sharing article and chapters. File paths of index.html are redundant.

Details

Add a new config field rewrite-to-dir under [output.html]. This field accepts Vec<String> of filename. Take a look for an example:

[output.html]
mathjax-support = true
no-section-label = true
rewrite-to-dir = ["index.html"]

This configuration only affects TOC and navigation without touch any actual contents.

[Michael-F-Bryan]

This sounds like a pretty good idea. I usually write the "main" chapter for each level as index.md just so it'll be accessible at /. I do have a couple questions though.

How should this interact with #685? Ideally I should be able to write the top level item in each directory as README.md and then have it accessible as /.

Also something to keep in mind is this blah/index.html => blah/ behaviour only works when you're viewing the file on a server. If you read the book from file (e.g. rustup doc --book, mdbook build --open) clicking on a blah/ link will present you with the contents of the blah directory instead of a rendered blah/README.md file...

[weihanglo]

Sorry, I didn't think of reading books from file system. I'll close this pull request. Thanks for your review!