showLineNumbers now loaded from config file

I would expect `showLineNumbers` to be loaded from config file.

I guess `showLineNumbers` should be added to `func validateOptions` but im not sure I fully understand the code.

Start by reading func validateOptions and tracing how the config file options are loaded. Confirm where showLineNumbers is validated or omitted, then verify that setting it in the config file changes rendered output as expected.

Written by the indexing model from the issue text.