Add clear_renderers / clear_preprocessors

[dalance]

Now Renderer and Preprocessor can be added by with_renderer and with_preprocessor.
But there is no way to disable the existing renderers and preprocessors.
So this PR add clear_renderers and clear_preprocessors.
This addition enables the customization of renderer and preprocessor without modifying the original book.toml.

[ehuss]

Can you say more about why you can't modify the Config?

[dalance]

@ehuss

I want to use this API for Rust documents translation project: https://rust-lang-translations.org .

In most cases, the maintainers of the original version are negative to modify their repository for translation. Keeping forks for translation takes a lot of time because these documents are updated frequently.

So I need the way to customize renderer/preprocesser without modifing the original TOML.

[ehuss]

If you are using the API, you should be able to load the config with Config::from_disk. Then modify the config, for example:

    if let Some(renderers) = config.get_mut("output").and_then(|o| o.as_table_mut()) {
        renderers.clear();
    }

and then use MdBook::load_with_config with whatever customizations you want. Does that work for you?

[dalance]

Thank you for your suggestion.
I missed load_with_config.