Use case: when trying to `mdbook test` a file that has many `include`
directives, and a test fails, the line numbers in the `rustdoc` output
don't match the line numbers in the original markdown file.
Turning on the markdown renderer implemented here lets you see what is
being passed to `rustdoc` by saving the markdown after the preprocessors
have run.
This renderer could be helpful for debugging many preprocessors, but
it's probably not useful in the general case, so it's turned off by
default.
* Added a windows-specific `tee` equivalent
* Changed how the windows "tee" command gets quoted
* Temporarily disabled the backends_receive_render_context_via_stdin test
* Added a mechanism for creating alternate backends
* Added a CmdRenderer and the ability to have multiple renderers
* Made MDBook::load() autodetect renderers
* Added a couple methods to RenderContext
* Converted RenderContext.version to a String
* Made sure all alternate renderers are invoked as `mdbook-*`
* Factored out the logic for determining which renderer to use
* Added tests for renderer detection
* Made it so `mdbook test` works on the book-example again
* Updated the "For Developers" docs
* Removed `[output.epub]` from the example book's book.toml
* Added a bit more info on how backends should work
* Added a `destination` key to the RenderContext
* Altered how we wait for an alternate backend to finish
* Refactored the Renderer trait to not use MDBook and moved livereload to the template
* Moved info for developers out of the book.toml format chapter
* MOAR docs
* MDBook::build() no longer takes &mut self
* Replaced a bunch of println!()'s with proper log macros
* Cleaned up the build() method and backend discovery
* Added a couple notes and doc-comments
* Found a race condition when backends exit really quickly
* Added support for backends with arguments
* Fixed a funny doc-comment
