Throw an error when unknown key-value arguments are passed with -D

[langston-barrett]

Description

When attempting to use the golang codegen with the option withXml=true, I mistakenly typed withXML (#490). My error was silently ignored and I spent a few minutes trying to figure out why it wasn't working. It might be more user-friendly to throw an error.

openapi-generator version

Docker

Command line used for generation

sudo docker run --rm -v ${PWD}:/local \
     openapitools/openapi-generator-cli generate \
     -i /local/swagger.yml \
     -g go \
     -o /local/generated/client \
     -D withXML=true

vs

sudo docker run --rm -v ${PWD}:/local \
     openapitools/openapi-generator-cli generate \
     -i /local/swagger.yml \
     -g go \
     -o /local/generated/client \
     -D withXml=true

Steps to reproduce

Run the above incorrect command

Related issues/PRs

Suggest a fix/enhancement

Throw an error!