sorting breaks anchors & aliases

[lollipopman]

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)