* 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
