Port to Legivel YAML parser

There is a nice [Legivel](https://github.com/fjoppe/Legivel) library. I suggest to port Nightwatch YAML parsing to this library, because it should feel more native for F# code.

Locate Nightwatch's existing YAML parsing entry point and compare its behavior with the Legivel library linked in the issue. Done means Nightwatch uses Legivel for YAML parsing while preserving the existing configuration behavior; the payload names no files or tests to run.

Written by the indexing model from the issue text.