support custom writers as format keys

[cscheid]

This should work:

---
format:
  customformat.lua: default
---

Instead we get a validation error. With validate-yaml: false everything works as expected.

[cscheid]

Unresolved question: what's the format schema that a custom format should respect? Plain md? Something else?

[dragonstyle]

Internally, it now inherits from 'unknown format' which has no customization over the base format (with the extension of a default txt extension on the output file). Not sure best approach for completions in this case, since we really don't know what keys will apply to writer...

[cscheid]

Resolution: anything goes.