Exposed the MDBook::execute_build_process() method to 3rd parties

[Michael-F-Bryan]

I think it's pretty common for 3rd party backends to expose some sort of --standalone flag so you could (for example) run mdbook-linkcheck --standalone user-guide to load the book and execute just that renderer.

Exposing the MDBook::execute_build_process() means we can replace this manual construction of a RenderContext and invoking the rendering system:

    // get a `RenderContext`, either from stdin (because we're used as a plugin)
    // or by instrumenting MDBook directly (in standalone mode).
    let ctx: RenderContext = if args.standalone {
        let md = MDBook::load(dunce::canonicalize(&args.root)?)
            .map_err(SyncFailure::new)?;
        let destination = md.build_dir_for("linkcheck");
        RenderContext::new(md.root, md.book, md.config, destination)
    } else {
        serde_json::from_reader(io::stdin())
            .context("Unable to parse RenderContext")?
    };

    let cache_file = ctx.destination.join("cache.json");
    mdbook_linkcheck::run(&cache_file, args.colour, &ctx)

With something like this:

let renderer = Renderer::new();

if is_standalone {
    let book = MDBook::load(&args.book_directory)?;
    book.execute_build_process(&renderer)?;
} else {
    let ctx = RenderContext::from_json(io::stdin())?;
    renderer.run(&ctx)?;
}

EDIT: Looking more closely, I think mdbook-linkcheck isn't even doing things correctly here. We forget to run all applicable preprocessors over the book because it's trying to execute the rendering pipeline manually.

[ehuss]

Seems reasonable.