CLI arg --help

[purefan]

I had to find the port argument by reading https://github.com/mulesoft/api-designer/blob/92e4717e4093614cd3b0dbde2d7b8ac2a096a804/bin/api-designer.js

it would be nice (and up to a point expected) to be able to call api-designer --help and see a reasonable list of options, even if its just the port number