YAML stub mapping support

[tomakehurst]

Proposal

Everyone absolutely loves YAML, so let's add support for reading and writing YAML stub mappings.

If possible (since YAML is a JSON superset) it would be great to simply extend support for parsing stub definitions to YAML.
If not possible (because this introduces edge cases) then we should pick the appropriate parser based on filename extension or the Content-Type header's MIME type.

An extra startup parameter should be added to set the default format when writing stub mapping files.

References

No response