- 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)