Fix output when using "--json" or "-j" option

[Lyrkan]

This PR fixes #204 by detecting the --json and -j options and:

• removing some console.log calls
• removing the FriendlyErrorsWebpackPlugin
• Using default stats values

As said in that comment of the related issue the --silent option has to be used with yarn run encore or yarn encore and must be placed before the name of the script.

For instance:

# Works
./node_modules/.bin/encore production --json
yarn run --silent encore production  --json
yarn --silent encore production  --json

# Doesn't work (some lines being added by yarn)
yarn run encore production --json
yarn encore production --json

# Doesn't work (since Webpack doesn't know the --silent option)
yarn run encore production --silent --json
yarn encore production --silent --json

Edit: Forgot to add a test yesterday, should be good now :)

[weaverryan]

Thanks @Lyrkan :). Sorry for the very slow merge - will start catching up soon (this was a very easy merge!)