use yaml_metadata_block for preserve_yaml feature when possible

[cderv]

This will fix #2118 and modify a bit how md_document() is working internally

• We use a standalone (--standalone) document if preserve_yaml = TRUE
• We try to add / remove the extension yaml_metadata_block depending on the value of variant and preserve_yaml. This is because some formats have the extension set or unset by default, and also depending of the pandoc version.
• We add some mechanism to handle parsing and setting those extensions. They could be externalize at some point to be used in other functions.

Doing this should handle correctly most of the case. Let's add that --standalone flag will produce this header, and that some format will have special header if not yaml:

• markdown will have a pandoc_title_block if not a YAML block
• markdown_mmd will have mmd_title_block if YAML block is unset.

For now, this is the behavior we apply for all variants:

• md_document(preserve_yaml = TRUE) will always produce a YAML block.
• md_document(preserve_yaml = FALSE) will always produce no YAML block.

And this is currently not working which is the main issue of #2118

Should we do something simpler or does it seems like a right thing to do ?

[yihui]

Looks good. Thanks!

[yihui]

Please feel free to merge after adding a news item. Thanks!

[cderv]

Do we want to merge this for the next release mid-august planned for the IDE ?
We discussed last time to only release current change.

I am hesitant as there would not be much time for this to live in dev version in case I missed something.
How do you feel about that ?

[yihui]

I don't feel this is a risky change that would affect users widely, so I'm okay with merging it into the next release.