replace js-yaml with yaml

[mkahvi]

Implements #80

This mimics js-yaml output as closely as possible to reduce output drift and needing to reconfigure things.

Notable flaws:

• Old yaml options obviously do not work (e.g. sortKeys)
  • yaml has features for this, but it is not just a boolean and it is by different name
• js-yaml "smartly" used block folding for long strings while yaml has no such feature. It uses block folding only if newlines are present.
  • This affects only first two lines of large text blocks so it isn't too terrible.

I'm fine if this PR is thrown away.