Sometimes we need to transform the ast to make it easier to print, but it's currently done in the parser (markdown and yaml), which leads the output ast harder to use for external users (custom parser), adding `printer.preprocess` can solve this issue.
And also this way we could move the trailing newline for json from `ast-to-doc.js` to `language-js`.
- upgrade to `yaml@1.0.0-rc.8` and `yaml-unist-parser@1.0.0-rc.4`
- refactor some logic since the AST has slightly changed (ikatyang/yaml-unist-parser#82)
- unmatched aliases are now errors since it may introduce invalid AST from `yaml`
- rewrite the document separator (`...`/`---`) logic, this fixes some cases where it can use `---` but we printed `...`
- removed some unnecessary duplicate trailing newline
- trailing comments on `document` (`... #comment`) and `documentHead` (`--- #comment`) are preserved (i.e. they won't be moved somewhere)
Ika