Assign default classes for code blocks of source, output, and so on with html_document

[atusy]

I'm currently working on this issue at https://github.com/atusy/rmarkdown/tree/default-class .

transferred from yihui/knitr#1730 .

What about code blocks generated by chunks have one of the following classes chunk-source, chunk-output, chunk-message, chunk-warning, or chunk-error by default?

This enables easier and precise selections in CSS and JavaScript, which is friendly both for developers and users.

I already implemented it, but am having difficulty in passing tests (atusy/knitr@54db8c4).
So I want to ask if @yihui like my idea and if it is worth working on to pass tests.

Example applications

Cumbersome CSS selections in a PR #1596 can be simplified (https://github.com/atusy/rmarkdown/blob/eb389d721b9f6cddd092234057c98150f70f898f/inst/rmd/h/default.html#L27-L42)

Selection of code blocks to fold by code_folding can be generalized.
Currently, folding is performed on code blocks with certain language classes, which may cause unexpected behavior (#1603).
If folding is performed on code blocks with chunk-source, the unexpected behavior can be suppressed.
In addition, we don't have to update codefolding.js everytime we find additional language to be folded.

There's a request on folding results #1453.
I am currently working on this issue, and seems to be easy.
However, a problem is that choice of folding button, which is currently code and hide.
For folding results, the choice is better to be output and hide.
I want to generate choices conditionally by detecting the code blocks have chunk-source class or chunk-output class.

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