Deprecate JSON formatter

[aslakhellesoy]

The JSON formatter is implemented in cucumber.js, cucumber-ruby and cucumber-jvm. It's become the defacto standard for machine-readable output from Cucumber, but it has several flaws:

• There is no schema for the output, so different platforms implement it inconsistently, causing problems for consumers
• It sometimes causes out of memory problems for large suites
• It does a poor job at representing results for background and scenario outlines
• Results cannot be consumed until Cucumber has finished

We should deprecate this in version 7 and encourage users and tool vendors to migrate to a new protobuf formatter outputting cucumber-messages.

The protobuf formatter addresses all of the problems with the json formatter:

• There is a strict schema documented here.
• It's a streaming protocol, so large memory consumption is less likely to be a problem
• It separates results from the AST and does a better job at representing results for backgrounds and scenario outlines
• Results can be consumed before Cucumber finishes (useful for slow suites)

See a89eae3 for more details.

[RusmanCool]

@aslakhellesoy "cucumber-messages." link is not working anymore. Also, how will the proposed solution work with a tool like "cucumber-html-reporter"?