Allow running without a configuration file, and add mnemonics -i / -o for --input / --output

[gilsdav]

Hello, is it possible to have same CLI options as ng-swagger-gen ?
I want to use same config only by switching the library.

I want to use it like this : ng-openapi-gen --gen-confi -i https://gist.githubusercontent.com/kinlane/43934f44fd591a6ee59a45267d9e3066/raw/47f6d7115dd17749c408c55861638ffc22d995f4/sample-openapi-30.yaml -o ./src/api -c ng-swagger-gen-api.json

Thank's in advance.

[luisfpg]

The command-line arguments in come directly from the options JSON schema.
I could, however, add a mapping for some shorter mnemonics for the input and output options, would be handy.