Add netlify --json flag option

[JakubKoralewski]

My attempt at #1206. I don't know Ruby much or the codebase. This works for me at the moment, but is probably somehow broken. I wouldn't be offended if someone wanted to do this differently.

I couldn't get Ruby, bundle, rspec to work locally so I tested it by running it on my deployment 🤭. Although it turned out Travis automatically runs tests for forks of dpl (?) so I used that as well.

Initially the idea was to export an environment variable, but it was not accessible in the after_deploy script. The current solution is to write a ./NETLIFY_DEPLOY_JSON_ID_<SITE_ID>.json file. If someone deploys the whole repo I guess this could be a problem, I guess home directory would be better?

Turns out it's not being saved in $TRAVIS_BUILD_DIR as I expected but somewhere else? Does anyone know where I can find that file 🥺 ? Ok it turns out it is being saved to wherever you cd inside before_deploy which makes sense.

[JakubKoralewski]

I've chosen to use an echo "..." > instead of write_file cause it didn't work.