DefaultPlaceholder does not apply placeholderClassName and placeholderStyle

This is a bug. The doc states that Editor accepts placeholderClassName and placeholderStyle props but they are not applied since https://github.com/ianstormtaylor/slate/pull/1253

I think this is an easy fix, DefaultComponent just need to read them and set the class / merge the style.

I can create a PR for this, but I am unsure if the docs (and editor.js, as the props are still defined there) are a leftover from the above PR and should be removed in favor of custom placeholder, or if they need to be added back to DefaultComponent. I would prefer the latter as this seems a common usecase and the implementation is quite simple.

[renchap]

See an example here: https://www.talegraph.com/tales/hX2bWZixOK?d

When editing the chapter’s title, I want the placeholder to be rendered like the normal text, but in grey, like this:

It might take multiple lines, I do not care but it needs to stay in the grey column.

I can definitely use a custom renderPlaceholder function and I understand that you want to keep the core simple, but in this case you need to remove mention of placeholderClassName and placeholderStyle in code/docs, and probably add an example with a custom placeholder as it is currently quite hard to figure it out from the docs/examples.

[ianstormtaylor]

This should be solved these days by implementing your own renderPlaceholder prop to render the placeholder with whatever extra styles you’d like.