sorting breaks anchors & aliases

Sorting yaml documents may break anchors or aliases, for example:

orig:
```
b_def: &domain 'example.com'
a_alias: *domain
```
post yamlfmt:
```
---
a_alias: *domain
b_def: &domain 'example.com'
```

parse:
```
$ ruby -e 'require "yaml"; YAML.load_file("test3.yaml")'
<snip> Unknown alias: domain (Psych::BadAlias)
```