silence swagger-diff execution if successful

[mhuisman]

issue ref: #22

simply put, silence error and warn reporting to stdout if there aren't any errors or warns to report

[mhuisman]

I'm also tempted to modify this line:

let output = 'swagger-diff\n';

to this:

let output = '';

in order to make successful output truly silent. have refrained from doing so as I wanted to get your feedback first.

[mhuisman]

in retrospect, I'm tempted to make a -s --silent Run silently if there are no errors detected help option and rework this a little.

[mhuisman]

closing stale PR